The C specification mandates that new keywords use _Keyword naming conventions to ensure backwards compatability by not overriding potentially existing identifiers in codebases. That is why the C specification has reserved identifiers that begin with an underscore and either an uppercase letter or another underscore.

Typically, a <stdkeyword.h> header is included that contain macros to provide the lowercased variants. I.E., this is how _Bool was implemented; <stdbool.h> provides the lowercased `bool` variant.