diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -1300,6 +1300,17 @@ always available.
    Note that as an exception is propagated down the chain of callers, an
    ``'exception'`` event is generated at each level.
 
+   For more fine-grained usage, it's possible to set a trace function by
+   assigning ``frame.f_trace = tracefunc`` explicitly, rather than relying on
+   it being set indirectly via the return value from an already installed
+   trace function. This is also required for activating the trace function on
+   the current frame, which :func:`settrace` doesn't do. Note that in order
+   for this to work, a global tracing function must have been installed
+   with :func:`settrace` in order to enable the runtime tracing machinery,
+   but it doesn't need to be the same tracing function (e.g. it could be a
+   low overhead tracing function that simply returns ``None`` to disable
+   itself immediately on each frame).
+
    For more information on code and frame objects, refer to :ref:`types`.
 
    .. audit-event:: sys.settrace "" sys.settrace

diff --git a/Misc/NEWS.d/next/Documentation/2019-08-24-12-59-06.bpo-37937.F7fHbt.rst b/Misc/NEWS.d/next/Documentation/2019-08-24-12-59-06.bpo-37937.F7fHbt.rst
@@ -0,0 +1 @@
+Mention ``frame.f_trace`` in :func:`sys.settrace` docs.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Mention ``frame.f_trace`` in :func:`sys.settrace` docs.