diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index 6a2168c..8aaedf3 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -17298,18 +17298,29 @@ Flag `using namespace` at global scope in a header file.
 
 To avoid files being `#include`d several times.
 
+In order to avoid include guard collisions, do not just name the guard after the filename.
+Be sure to also include a key and good differentiator, such as the name of library or component
+the header file is part of.
+
 ##### Example
 
     // file foobar.h:
-    #ifndef FOOBAR_H
-    #define FOOBAR_H
+    #ifndef LIBRARY_FOOBAR_H
+    #define LIBRARY_FOOBAR_H
     // ... declarations ...
-    #endif // FOOBAR_H
+    #endif // LIBRARY_FOOBAR_H
 
 ##### Enforcement
 
 Flag `.h` files without `#include` guards.
 
+#####  Note
+
+Some implementations offer vendor extensions like `#pragma once` as alternative to include guards.
+It is not standard and it is not portable.  It injects the hosting machine's filesystem semantics
+into your program, in addition to locking you down to a vendor.
+Our recommendation is to write in ISO C++: See [rule P.2]((#Rp-Cplusplus).
+
 ### <a name="Rs-cycles"></a>SF.9: Avoid cyclic dependencies among source files
 
 ##### Reason