diff --git a/cibuildwheel/__main__.py b/cibuildwheel/__main__.py
index ab1aa654..7099bc8f 100644
--- a/cibuildwheel/__main__.py
+++ b/cibuildwheel/__main__.py
@@ -48,26 +48,34 @@ def strtobool(val):
 def main():
     parser = argparse.ArgumentParser(
         description='Build wheels for all the platforms.',
-        epilog=('Most options are supplied via environment variables. '
-                'See https://github.com/joerick/cibuildwheel#options for info.'))
+        epilog='''
+            Most options are supplied via environment variables.
+            See https://github.com/joerick/cibuildwheel#options for info.
+        ''')
 
     parser.add_argument('--platform',
                         choices=['auto', 'linux', 'macos', 'windows'],
                         default=os.environ.get('CIBW_PLATFORM', 'auto'),
-                        help=('Platform to build for. For "linux" you need docker running, on Mac '
-                              'or Linux. For "macos", you need a Mac machine, and note that this '
-                              'script is going to automatically install MacPython on your system, '
-                              'so don\'t run on your development machine. For "windows", you need to '
-                              'run in Windows, and it will build and test for all versions of '
-                              'Python. Default: auto.'))
+                        help='''
+                            Platform to build for. For "linux" you need docker running, on Mac
+                            or Linux. For "macos", you need a Mac machine, and note that this
+                            script is going to automatically install MacPython on your system,
+                            so don't run on your development machine. For "windows", you need to
+                            run in Windows, and it will build and test for all versions of
+                            Python. Default: auto.
+                        ''')
     parser.add_argument('--output-dir',
                         default=os.environ.get('CIBW_OUTPUT_DIR', 'wheelhouse'),
                         help='Destination folder for the wheels.')
-    parser.add_argument('project_dir',
+    parser.add_argument('package_dir',
                         default='.',
                         nargs='?',
-                        help=('Path to the project that you want wheels for. Default: the current '
-                              'directory.'))
+                        help='''
+                            Path to the package that you want wheels for. Must be a subdirectory of
+                            the working directory. When set, the working directory is still
+                            considered the 'project' and is copied into the Docker container on
+                            Linux. Default: the working directory.
+                        ''')
 
     parser.add_argument('--print-build-identifiers',
                         action='store_true',
@@ -104,7 +112,7 @@ def main():
     test_command = get_option_from_environment('CIBW_TEST_COMMAND', platform=platform)
     test_requires = get_option_from_environment('CIBW_TEST_REQUIRES', platform=platform, default='').split()
     test_extras = get_option_from_environment('CIBW_TEST_EXTRAS', platform=platform, default='')
-    project_dir = args.project_dir
+    package_dir = args.package_dir
     before_build = get_option_from_environment('CIBW_BEFORE_BUILD', platform=platform)
     build_verbosity = get_option_from_environment('CIBW_BUILD_VERBOSITY', platform=platform, default='')
     build_config, skip_config = os.environ.get('CIBW_BUILD', '*'), os.environ.get('CIBW_SKIP', '')
@@ -147,8 +155,8 @@ def main():
     # This needs to be passed on to the docker container in linux.py
     os.environ['CIBUILDWHEEL'] = '1'
 
-    if not os.path.exists(os.path.join(project_dir, 'setup.py')):
-        print('cibuildwheel: Could not find setup.py at root of project', file=sys.stderr)
+    if not os.path.exists(os.path.join(package_dir, 'setup.py')):
+        print('cibuildwheel: Could not find setup.py at root of package', file=sys.stderr)
         exit(2)
 
     if args.print_build_identifiers:
@@ -189,7 +197,7 @@ def main():
         manylinux_images = None
 
     build_options = BuildOptions(
-        project_dir=project_dir,
+        package_dir=package_dir,
         output_dir=output_dir,
         test_command=test_command,
         test_requires=test_requires,
@@ -201,7 +209,7 @@ def main():
         environment=environment,
         before_test=before_test,
         dependency_constraints=dependency_constraints,
-        manylinux_images=manylinux_images
+        manylinux_images=manylinux_images,
     )
 
     # Python is buffering by default when running on the CI platforms, giving problems interleaving subprocess call output with unflushed calls to 'print'
diff --git a/cibuildwheel/linux.py b/cibuildwheel/linux.py
index a28c0a49..04b2a620 100644
--- a/cibuildwheel/linux.py
+++ b/cibuildwheel/linux.py
@@ -96,12 +96,18 @@ def build(options: BuildOptions):
         ('pp', 'manylinux_x86_64', options.manylinux_images['pypy_x86_64']),
     ]
 
+    if not os.path.realpath(options.package_dir).startswith(os.path.realpath('.')):
+        raise Exception('package_dir must be inside the working directory')
+
+    container_package_dir = os.path.join('/project', os.path.relpath(options.package_dir, '.'))
+
     for implementation, platform_tag, docker_image in platforms:
         platform_configs = [c for c in python_configurations if c.identifier.startswith(implementation) and c.identifier.endswith(platform_tag)]
         if not platform_configs:
             continue
 
         container_name = 'cibuildwheel-{}'.format(uuid.uuid4())
+
         try:
             call(['docker', 'create',
                   '--env', 'CIBUILDWHEEL',
@@ -111,9 +117,7 @@ def build(options: BuildOptions):
                   docker_image,
                   '/bin/bash'])
 
-            call(['docker', 'cp',
-                  os.path.abspath(options.project_dir) + '/.',
-                  container_name + ':/project'])
+            call(['docker', 'cp', '.', container_name + ':/project'])
 
             call(['docker', 'start', container_name])
 
@@ -165,7 +169,7 @@ def build(options: BuildOptions):
                         # Build the wheel
                         rm -rf /tmp/built_wheel
                         mkdir /tmp/built_wheel
-                        pip wheel . -w /tmp/built_wheel --no-deps {build_verbosity_flag}
+                        pip wheel {package_dir} -w /tmp/built_wheel --no-deps {build_verbosity_flag}
                         built_wheel=(/tmp/built_wheel/*.whl)
 
                         # repair the wheel
@@ -234,13 +238,14 @@ def build(options: BuildOptions):
                         done
                     '''.format(
                         config_python_bin=config.path + '/bin',
+                        package_dir=container_package_dir,
                         test_requires=' '.join(options.test_requires),
                         test_extras=options.test_extras,
                         test_command=shlex.quote(
-                            prepare_command(options.test_command, project='/project') if options.test_command else ''
+                            prepare_command(options.test_command, project='/project', package=container_package_dir) if options.test_command else ''
                         ),
                         before_build=shlex.quote(
-                            prepare_command(options.before_build, project='/project') if options.before_build else ''
+                            prepare_command(options.before_build, project='/project', package=container_package_dir) if options.before_build else ''
                         ),
                         build_verbosity_flag=' '.join(get_build_verbosity_extra_flags(options.build_verbosity)),
                         repair_command=shlex.quote(
@@ -250,7 +255,7 @@ def build(options: BuildOptions):
                         uid=os.getuid(),
                         gid=os.getgid(),
                         before_test=shlex.quote(
-                            prepare_command(options.before_test, project='/project') if options.before_test else ''
+                            prepare_command(options.before_test, project='/project', package=container_package_dir) if options.before_test else ''
                         ),
                         dependency_install_flags='-c /constraints.txt' if options.dependency_constraints else '',
                     )
@@ -268,12 +273,12 @@ def build(options: BuildOptions):
             call(['docker', 'rm', '--force', '-v', container_name])
 
 
-def troubleshoot(project_dir, error):
+def troubleshoot(package_dir, error):
     if (isinstance(error, subprocess.CalledProcessError) and 'exec' in error.cmd):
         # the bash script failed
         print('Checking for common errors...')
         so_files = []
-        for root, dirs, files in os.walk(project_dir):
+        for root, dirs, files in os.walk(package_dir):
             for name in files:
                 _, ext = os.path.splitext(name)
                 if ext == '.so':
diff --git a/cibuildwheel/macos.py b/cibuildwheel/macos.py
index 798cb493..599e9b23 100644
--- a/cibuildwheel/macos.py
+++ b/cibuildwheel/macos.py
@@ -168,7 +168,6 @@ def setup_python(python_configuration, dependency_constraint_flags, environment)
 
 
 def build(options: BuildOptions):
-    abs_project_dir = os.path.abspath(options.project_dir)
     temp_dir = tempfile.mkdtemp(prefix='cibuildwheel')
     built_wheel_dir = os.path.join(temp_dir, 'built_wheel')
     repaired_wheel_dir = os.path.join(temp_dir, 'repaired_wheel')
@@ -186,14 +185,14 @@ def build(options: BuildOptions):
 
         # run the before_build command
         if options.before_build:
-            before_build_prepared = prepare_command(options.before_build, project=abs_project_dir)
+            before_build_prepared = prepare_command(options.before_build, project='.', package=options.package_dir)
             call(before_build_prepared, env=env, shell=True)
 
         # build the wheel
         if os.path.exists(built_wheel_dir):
             shutil.rmtree(built_wheel_dir)
         os.makedirs(built_wheel_dir)
-        call(['pip', 'wheel', abs_project_dir, '-w', built_wheel_dir, '--no-deps'] + get_build_verbosity_extra_flags(options.build_verbosity), env=env)
+        call(['pip', 'wheel', options.package_dir, '-w', built_wheel_dir, '--no-deps'] + get_build_verbosity_extra_flags(options.build_verbosity), env=env)
         built_wheel = glob(os.path.join(built_wheel_dir, '*.whl'))[0]
 
         # repair the wheel
@@ -229,7 +228,7 @@ def build(options: BuildOptions):
             call(['which', 'python'], env=virtualenv_env)
 
             if options.before_test:
-                before_test_prepared = prepare_command(options.before_test, project=abs_project_dir)
+                before_test_prepared = prepare_command(options.before_test, project='.', package=options.package_dir)
                 call(before_test_prepared, env=virtualenv_env, shell=True)
 
             # install the wheel
@@ -242,7 +241,11 @@ def build(options: BuildOptions):
             # run the tests from $HOME, with an absolute path in the command
             # (this ensures that Python runs the tests against the installed wheel
             # and not the repo code)
-            test_command_prepared = prepare_command(options.test_command, project=abs_project_dir)
+            test_command_prepared = prepare_command(
+                options.test_command,
+                project=os.path.abspath('.'),
+                package=os.path.abspath(options.package_dir)
+            )
             call(test_command_prepared, cwd=os.environ['HOME'], env=virtualenv_env, shell=True)
 
             # clean up
diff --git a/cibuildwheel/util.py b/cibuildwheel/util.py
index 03fe6af5..64f561a1 100644
--- a/cibuildwheel/util.py
+++ b/cibuildwheel/util.py
@@ -107,7 +107,7 @@ class DependencyConstraints:
 
 
 BuildOptions = NamedTuple("BuildOptions", [
-    ("project_dir", str),
+    ("package_dir", str),
     ("output_dir", str),
     ("test_command", Optional[str]),
     ("test_requires", List[str]),
diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index db5fc48a..e6f14ed9 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -143,7 +143,6 @@ def setup_python(python_configuration, dependency_constraint_flags, environment)
 
 
 def build(options: BuildOptions):
-    abs_project_dir = os.path.abspath(options.project_dir)
     temp_dir = tempfile.mkdtemp(prefix='cibuildwheel')
     built_wheel_dir = os.path.join(temp_dir, 'built_wheel')
     repaired_wheel_dir = os.path.join(temp_dir, 'repaired_wheel')
@@ -165,14 +164,14 @@ def build(options: BuildOptions):
 
         # run the before_build command
         if options.before_build:
-            before_build_prepared = prepare_command(options.before_build, project=abs_project_dir)
+            before_build_prepared = prepare_command(options.before_build, project='.', package=options.package_dir)
             shell([before_build_prepared], env=env)
 
         # build the wheel
         if os.path.exists(built_wheel_dir):
             shutil.rmtree(built_wheel_dir)
         os.makedirs(built_wheel_dir)
-        shell(['pip', 'wheel', abs_project_dir, '-w', built_wheel_dir, '--no-deps'] + get_build_verbosity_extra_flags(options.build_verbosity), env=env)
+        shell(['pip', 'wheel', options.package_dir, '-w', built_wheel_dir, '--no-deps'] + get_build_verbosity_extra_flags(options.build_verbosity), env=env)
         built_wheel = glob(os.path.join(built_wheel_dir, '*.whl'))[0]
 
         # repair the wheel
@@ -213,7 +212,11 @@ def build(options: BuildOptions):
             shell(['which', 'python'], env=virtualenv_env)
 
             if options.before_test:
-                before_test_prepared = prepare_command(options.before_test, project=abs_project_dir)
+                before_test_prepared = prepare_command(
+                    options.before_test,
+                    project='.',
+                    package=options.package_dir
+                )
                 shell([before_test_prepared], env=virtualenv_env)
 
             # install the wheel
@@ -226,7 +229,11 @@ def build(options: BuildOptions):
             # run the tests from c:\, with an absolute path in the command
             # (this ensures that Python runs the tests against the installed wheel
             # and not the repo code)
-            test_command_prepared = prepare_command(options.test_command, project=abs_project_dir)
+            test_command_prepared = prepare_command(
+                options.test_command,
+                project=os.path.abspath('.'),
+                package=os.path.abspath(options.package_dir)
+            )
             shell([test_command_prepared], cwd='c:\\', env=virtualenv_env)
 
             # clean up
diff --git a/docs/options.md b/docs/options.md
index c17e4933..8ecaaf23 100644
--- a/docs/options.md
+++ b/docs/options.md
@@ -199,7 +199,7 @@ A shell command to run before building the wheel. This option allows you to run
 
 If dependencies are required to build your wheel (for example if you include a header from a Python module), set this to `pip install .`, and the dependencies will be installed automatically by pip. However, this means your package will be built twice - if your package takes a long time to build, you might wish to manually list the dependencies here instead.
 
-The active Python binary can be accessed using `python`, and pip with `pip`; `cibuildwheel` makes sure the right version of Python and pip will be executed. `{project}` can be used as a placeholder for the absolute path to the project's root and will be replaced by `cibuildwheel`.
+The active Python binary can be accessed using `python`, and pip with `pip`; `cibuildwheel` makes sure the right version of Python and pip will be executed. The placeholder `{package}` can be used here; it will be replaced by the path to the package being built by `cibuildwheel`.
 
 The command is run in a shell, so you can write things like `cmd1 && cmd2`.
 
@@ -217,8 +217,11 @@ CIBW_BEFORE_BUILD: pip install pybind11
 # chain commands using &&
 CIBW_BEFORE_BUILD: yum install -y libffi-dev && pip install .
 
-# run a script that's inside your repo
+# run a script that's inside your project
 CIBW_BEFORE_BUILD: bash scripts/prepare_for_build.sh
+
+# if cibuildwheel is called with a package_dir argument, it's available as {package}
+CIBW_BEFORE_BUILD: "{package}/bin/prepare_for_build.sh"
 ```
 
 
@@ -348,7 +351,10 @@ CIBW_DEPENDENCY_VERSIONS: ./constraints.txt
 ### `CIBW_TEST_COMMAND` {: #test-command}
 > Execute a shell command to test each built wheel
 
-Shell command to run tests after the build. The wheel will be installed automatically and available for import from the tests. `{project}` can be used as a placeholder for the absolute path to the project's root and will be replaced by `cibuildwheel`.
+Shell command to run tests after the build. The wheel will be installed automatically and available for import from the tests. To ensure the wheel is imported by your tests (instead of your source copy), tests are run from a different directory. Use the placeholders `{project}` and `{package}` when specifying paths in your project.
+
+- `{project}` is an absolute path to the project root - the working directory where cibuildwheel was called.
+- `{package}` is the path to the package being built - the `package_dir` argument supplied to cibuildwheel on the command line.
 
 The command is run in a shell, so you can write things like `cmd1 && cmd2`.
 
@@ -361,8 +367,8 @@ Platform-specific variants also available:<br/>
 # run the project tests against the installed wheel using `nose`
 CIBW_TEST_COMMAND: nosetests {project}/tests
 
-# run the project tests using `pytest`
-CIBW_TEST_COMMAND: pytest {project}/tests
+# run the package tests using `pytest`
+CIBW_TEST_COMMAND: pytest {package}/tests
 ```
 
 
@@ -411,7 +417,7 @@ CIBW_TEST_EXTRAS: test,qt
 A shell command to run in **each** test virtual environment, before your wheel is installed and tested. This is useful if you need to install a non pip package, change values of environment variables
 or perform multi step pip installation (e.g. installing `scikit-build` or `cython` before install test package)
 
-The active Python binary can be accessed using `python`, and pip with `pip`; `cibuildwheel` makes sure the right version of Python and pip will be executed. `{project}` can be used as a placeholder for the absolute path to the project's root and will be replaced by `cibuildwheel`.
+The active Python binary can be accessed using `python`, and pip with `pip`; `cibuildwheel` makes sure the right version of Python and pip will be executed. The placeholder `{package}` can be used here; it will be replaced by the path to the package being built by `cibuildwheel`.
 
 The command is run in a shell, so you can write things like `cmd1 && cmd2`.
 
@@ -456,13 +462,16 @@ CIBW_BUILD_VERBOSITY: 1
 ```text
 usage: cibuildwheel [-h] [--platform {auto,linux,macos,windows}]
                     [--output-dir OUTPUT_DIR] [--print-build-identifiers]
-                    [project_dir]
+                    [package_dir]
 
 Build wheels for all the platforms.
 
 positional arguments:
-  project_dir           Path to the project that you want wheels for.
-                        Default: the current directory.
+  package_dir           Path to the package that you want wheels for. Must be
+                        a subdirectory of the working directory. When set, the
+                        working directory is still considered the 'project'
+                        and is copied into the Docker container on Linux.
+                        Default: the working directory.
 
 optional arguments:
   -h, --help            show this help message and exit
@@ -473,13 +482,12 @@ optional arguments:
                         automatically install MacPython on your system, so
                         don't run on your development machine. For "windows",
                         you need to run in Windows, and it will build and test
-                        for all versions of Python at C:\PythonXX[-x64].
+                        for all versions of Python. Default: auto.
   --output-dir OUTPUT_DIR
                         Destination folder for the wheels.
   --print-build-identifiers
                         Print the build identifiers matched by the current
                         invocation and exit.
-
 ```
 
 <style>
diff --git a/test/13_subdir_package/bin/before_build.py b/test/13_subdir_package/bin/before_build.py
new file mode 100755
index 00000000..3bc8ab16
--- /dev/null
+++ b/test/13_subdir_package/bin/before_build.py
@@ -0,0 +1 @@
+print('before_build.py executed!')
diff --git a/test/13_subdir_package/cibuildwheel_test.py b/test/13_subdir_package/cibuildwheel_test.py
new file mode 100644
index 00000000..404811db
--- /dev/null
+++ b/test/13_subdir_package/cibuildwheel_test.py
@@ -0,0 +1,25 @@
+import os
+import utils
+
+project_dir = os.path.dirname(__file__)
+
+
+def test(capfd):
+    package_dir = os.path.join(project_dir, 'src', 'spam')
+    # build the wheels
+    actual_wheels = utils.cibuildwheel_run(project_dir, package_dir=package_dir, add_env={
+        'CIBW_BEFORE_BUILD': 'python {project}/bin/before_build.py',
+        'CIBW_TEST_COMMAND': 'python {package}/test/run_tests.py',
+        # this shouldn't depend on the version of python, so build only
+        # CPython 3.6
+        'CIBW_BUILD': 'cp36-*',
+    })
+
+    # check that the expected wheels are produced
+    expected_wheels = [w for w in utils.expected_wheels('spam', '0.1.0')
+                       if 'cp36' in w]
+    assert set(actual_wheels) == set(expected_wheels)
+
+    captured = capfd.readouterr()
+    assert "before_build.py executed!" in captured.out
+    assert "run_tests.py executed!" in captured.out
diff --git a/test/13_subdir_package/src/spam/setup.py b/test/13_subdir_package/src/spam/setup.py
new file mode 100644
index 00000000..637d7ff0
--- /dev/null
+++ b/test/13_subdir_package/src/spam/setup.py
@@ -0,0 +1,8 @@
+from setuptools import Extension, setup
+
+
+setup(
+    name="spam",
+    ext_modules=[Extension('spam', sources=['spam.c'])],
+    version="0.1.0",
+)
diff --git a/test/13_subdir_package/src/spam/spam.c b/test/13_subdir_package/src/spam/spam.c
new file mode 100644
index 00000000..d1ab0f22
--- /dev/null
+++ b/test/13_subdir_package/src/spam/spam.c
@@ -0,0 +1,48 @@
+#include <Python.h>
+
+static PyObject *
+spam_system(PyObject *self, PyObject *args)
+{
+    const char *command;
+    int sts;
+
+    if (!PyArg_ParseTuple(args, "s", &command))
+        return NULL;
+    sts = system(command);
+    return PyLong_FromLong(sts);
+}
+
+/* Module initialization */
+
+#if PY_MAJOR_VERSION >= 3
+    #define MOD_INIT(name) PyMODINIT_FUNC PyInit_##name(void)
+    #define MOD_DEF(m, name, doc, methods, module_state_size) \
+        static struct PyModuleDef moduledef = { \
+            PyModuleDef_HEAD_INIT, name, doc, module_state_size, methods, }; \
+        m = PyModule_Create(&moduledef);
+    #define MOD_RETURN(m) return m;
+#else
+    #define MOD_INIT(name) PyMODINIT_FUNC init##name(void)
+    #define MOD_DEF(m, name, doc, methods, module_state_size) \
+        m = Py_InitModule3(name, methods, doc);
+    #define MOD_RETURN(m) return;
+#endif
+
+static PyMethodDef module_methods[] = {
+    {"system", (PyCFunction)spam_system, METH_VARARGS, 
+     "Execute a shell command."},
+    {NULL}  /* Sentinel */
+};
+
+MOD_INIT(spam)
+{
+    PyObject* m;
+
+    MOD_DEF(m, 
+            "spam", 
+            "Example module", 
+            module_methods,
+            -1)
+
+    MOD_RETURN(m)
+}
diff --git a/test/13_subdir_package/src/spam/test/run_tests.py b/test/13_subdir_package/src/spam/test/run_tests.py
new file mode 100644
index 00000000..8104e1b3
--- /dev/null
+++ b/test/13_subdir_package/src/spam/test/run_tests.py
@@ -0,0 +1 @@
+print('run_tests.py executed!')
diff --git a/test/shared/utils.py b/test/shared/utils.py
index 0b463334..609ab9d7 100644
--- a/test/shared/utils.py
+++ b/test/shared/utils.py
@@ -41,13 +41,15 @@ def cibuildwheel_get_build_identifiers(project_path, env=None):
     return cmd_output.strip().split('\n')
 
 
-def cibuildwheel_run(project_path, env=None, add_env=None, output_dir=None):
+def cibuildwheel_run(project_path, package_dir='.', env=None, add_env=None, output_dir=None):
     '''
     Runs cibuildwheel as a subprocess, building the project at project_path.
 
     Uses the current Python interpreter.
 
     :param project_path: path of the project to be built.
+    :param package_dir: path of the package to be built. Can be absolute, or
+    relative to project_path.
     :param env: full environment to be used, os.environ if None
     :param add_env: environment used to update env
     :param output_dir: directory where wheels are saved. If None, a temporary
@@ -64,8 +66,9 @@ def cibuildwheel_run(project_path, env=None, add_env=None, output_dir=None):
 
     with TemporaryDirectoryIfNone(output_dir) as _output_dir:
         subprocess.check_call(
-            [sys.executable, '-m', 'cibuildwheel', '--output-dir', str(_output_dir), project_path],
+            [sys.executable, '-m', 'cibuildwheel', '--output-dir', str(_output_dir), package_dir],
             env=env,
+            cwd=project_path,
         )
         wheels = os.listdir(_output_dir)
     return wheels
diff --git a/unit_test/main_tests/conftest.py b/unit_test/main_tests/conftest.py
index 227b8df7..cd896915 100644
--- a/unit_test/main_tests/conftest.py
+++ b/unit_test/main_tests/conftest.py
@@ -18,7 +18,7 @@ class ArgsInterceptor:
         self.kwargs = kwargs
 
 
-MOCK_PROJECT_DIR = 'some_project_dir'
+MOCK_PACKAGE_DIR = 'some_package_dir'
 
 
 @pytest.fixture(autouse=True)
@@ -39,20 +39,20 @@ def mock_protection(monkeypatch):
 
 
 @pytest.fixture(autouse=True)
-def fake_project_dir(monkeypatch):
+def fake_package_dir(monkeypatch):
     '''
     Monkey-patch enough for the main() function to run
     '''
     real_os_path_exists = os.path.exists
 
     def mock_os_path_exists(path):
-        if path == os.path.join(MOCK_PROJECT_DIR, 'setup.py'):
+        if path == os.path.join(MOCK_PACKAGE_DIR, 'setup.py'):
             return True
         else:
             return real_os_path_exists(path)
 
     monkeypatch.setattr(os.path, 'exists', mock_os_path_exists)
-    monkeypatch.setattr(sys, 'argv', ['cibuildwheel', MOCK_PROJECT_DIR])
+    monkeypatch.setattr(sys, 'argv', ['cibuildwheel', MOCK_PACKAGE_DIR])
 
 
 @pytest.fixture(params=['linux', 'macos', 'windows'])
diff --git a/unit_test/main_tests/main_platform_test.py b/unit_test/main_tests/main_platform_test.py
index 9159586b..009fb1b4 100644
--- a/unit_test/main_tests/main_platform_test.py
+++ b/unit_test/main_tests/main_platform_test.py
@@ -4,7 +4,7 @@ import pytest
 
 from cibuildwheel.__main__ import main
 
-from conftest import MOCK_PROJECT_DIR  # noqa: I100
+from conftest import MOCK_PACKAGE_DIR  # noqa: I100
 
 
 def test_unknown_platform_non_ci(monkeypatch, capsys):
@@ -12,6 +12,7 @@ def test_unknown_platform_non_ci(monkeypatch, capsys):
     monkeypatch.delenv('BITRISE_BUILD_NUMBER', raising=False)
     monkeypatch.delenv('AZURE_HTTP_USER_AGENT', raising=False)
     monkeypatch.delenv('GITHUB_WORKFLOW', raising=False)
+    monkeypatch.delenv('CIBW_PLATFORM', raising=False)
 
     with pytest.raises(SystemExit) as exit:
         main()
@@ -25,6 +26,7 @@ def test_unknown_platform_non_ci(monkeypatch, capsys):
 def test_unknown_platform_on_ci(monkeypatch, capsys):
     monkeypatch.setenv('CI', 'true')
     monkeypatch.setattr(sys, 'platform', 'nonexistent')
+    monkeypatch.delenv('CIBW_PLATFORM', raising=False)
 
     with pytest.raises(SystemExit) as exit:
         main()
@@ -51,10 +53,10 @@ def test_platform_argument(platform, intercepted_build_args, monkeypatch):
 
     main()
 
-    assert intercepted_build_args.args[0].project_dir == MOCK_PROJECT_DIR
+    assert intercepted_build_args.args[0].package_dir == MOCK_PACKAGE_DIR
 
 
 def test_platform_environment(platform, intercepted_build_args, monkeypatch):
     main()
 
-    assert intercepted_build_args.args[0].project_dir == MOCK_PROJECT_DIR
+    assert intercepted_build_args.args[0].package_dir == MOCK_PACKAGE_DIR