diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index ce24f65..e80f6f7 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -21626,7 +21626,8 @@ ISO Standard, use lower case only and digits, separate words with underscores:
 * `vector`
 * `my_map`
 
-Avoid double underscores `__`.
+Avoid identifier names that contain double underscores `__` or that start with an underscore followed by a capital letter (e.g., `_Throws`).
+Such identifiers are reserved for the C++ implementation.
 
 ##### Example