[valleyer]

You're right, and I guess I've been breaking that rule for a while. What's the purpose there? The double-underbar and underbar-capital rules seem to be allowing for non-conflicting introduction of keywords. Is the single-underbar rule to protect standard library headers or something?

[uecker]

Reserved identifiers in general are not only for future use of the standard, but also for internal use of implementations and for extensions. The single-under bar rule is to give the programmer some part of the _... namespace which is otherwise reserved for the implementation. I.e. for use in macros, variables, or struct members. The intention is for use in hidden names in some API.