diff --git a/docs/c_cxx/README.md b/docs/c_cxx/README.md
index 960c5f3e47..eabfd289a6 100644
--- a/docs/c_cxx/README.md
+++ b/docs/c_cxx/README.md
@@ -4,4 +4,23 @@ This directory contains doxygen configuration to generate the C/C++ API docs for
 
 The actual generation is performed by a GitHub actions workflow: [publish-c-apidocs.yml](../../.github/workflows/publish-c-apidocs.yml).
 
-The workflow is currently manually triggered, and generates a PR to the gh-pages branch, for publication on https://onnxruntime.ai.
\ No newline at end of file
+The workflow is currently manually triggered, and generates a PR to the gh-pages branch, for publication on https://onnxruntime.ai.
+
+# C/C++ API Documentation Conventions
+
+## Handling API changes across versions
+When a new API is added or an existing API is changed, indicate this in the API's documentation with the Doxygen `\since` command.
+This lets readers know the availability and behavior in different versions.
+
+For a new API in version X, add `\since Version X.`. If an API's behavior changes in version Y, add `\since Version Y: <change description>`.
+Put each API change entry for a version on its own line.
+
+For example:
+```c++
+/**
+ * \brief My API function.
+ * \since Version X.
+ * \since Version Y: Does things differently.
+ */
+void MyApi();
+```