bpo-36487: Make C-API docs clear about what the main interpreter is. (gh-12666)
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index a0ac4d2..4985a1d 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1141,10 +1141,18 @@
While in most uses, you will only embed a single Python interpreter, there
are cases where you need to create several independent interpreters in the
-same process and perhaps even in the same thread. Sub-interpreters allow
-you to do that. You can switch between sub-interpreters using the
-:c:func:`PyThreadState_Swap` function. You can create and destroy them
-using the following functions:
+same process and perhaps even in the same thread. Sub-interpreters allow
+you to do that.
+
+The "main" interpreter is the first one created when the runtime initializes.
+It is usually the only Python interpreter in a process. Unlike sub-interpreters,
+the main interpreter has unique process-global responsibilities like signal
+handling. It is also responsible for execution during runtime initialization and
+is usually the active interpreter during runtime finalization. The
+:c:func:`PyInterpreterState_Main` funtion returns a pointer to its state.
+
+You can switch between sub-interpreters using the :c:func:`PyThreadState_Swap`
+function. You can create and destroy them using the following functions:
.. c:function:: PyThreadState* Py_NewInterpreter()