diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 725a01cb472..dc9ac70a0c0 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -234,3 +234,27 @@ If you are working on the CUDA code, here are some useful CUDA debugging tips:
 
 
 Hope this helps, and thanks for considering to contribute.
+
+## Caffe2 notes
+
+In 2018, we merged Caffe2 into the PyTorch source repository.  While the
+steady state aspiration is that Caffe2 and PyTorch share code freely,
+in the meantime there will be some separation.
+
+If you submit a PR to only PyTorch or only Caffe2 code, CI will only
+run for the project you edited.  The logic for this is implemented
+in `.jenkins/pytorch/dirty.sh` and `.jenkins/caffe2/dirty.sh`; you
+can look at this to see what path prefixes constitute changes.
+This also means if you ADD a new top-level path, or you start
+sharing code between projects, you need to modify these files.
+
+There are a few "unusual" directories which, for historical reasons,
+are Caffe2/PyTorch specific.  Here they are:
+
+- `CMakeLists.txt`, `Makefile`, `binaries`, `cmake`, `conda`, `modules`,
+  `scripts` are Caffe2-specific.  Don't put PyTorch code in them without
+  extra coordination.
+
+- `mypy*`, `requirements.txt`, `setup.py`, `test`, `tools` are
+  PyTorch-specific.  Don't put Caffe2 code in them without extra
+  coordination.