diff --git a/g3doc/implementation_basics.md b/g3doc/implementation_basics.md
index 114b7a7c92..933941a0d1 100644
--- a/g3doc/implementation_basics.md
+++ b/g3doc/implementation_basics.md
@@ -80,6 +80,8 @@ in the (slow) process of being removed from the codebase.
   
 * AsyncInvoker.
 
+* RecursiveCriticalSection. Try to use [webrtc::Mutex][6] instead, and don't recurse.
+
 
 
 [1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/units/timestamp.h;drc=b95d90b78a3491ef8e8aa0640dd521515ec881ca;l=29
@@ -87,3 +89,4 @@ in the (slow) process of being removed from the codebase.
 [3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/task_queue/task_queue_base.h;drc=1107751b6f11c35259a1c5c8a0f716e227b7e3b4;l=25
 [4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/callback_list.h;drc=54b91412de3f579a2d5ccdead6e04cc2cc5ca3a1;l=162
 [5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/task_utils/pending_task_safety_flag.h;drc=86ee89f73e4f4799b3ebcc0b5c65837c9601fe6d;l=117
+[6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/rtc_base/synchronization/mutex.h;drc=0d3c09a8fe5f12dfbc9f1bcd5790fda8830624ec;l=40