To be more precise, in Scheme it seems to be traditional to end this type of function with a '?'. In Common Lisp, depending on whether or not the rest of the function's name has a '-' in it or not, one ends it with -p or p (e.g one would have simple-string-p and stringp). (I think there may be an exception to this, but it escapes me at the moment. And I don't think this is followed strictly for certain functions which get automatically defined when using defstruct - which may even be the exceptions I'm thinking of.) I do not know about other Lisp dialects.