Older packages may not conform to this rule (no subdirectories) and for these there may be a hierarchy of subdirectories within the package directory itself. In these cases all ``externally visible'' include files will be found in one of the following standard places:

a subdirectory with the same name as the package, or
a subdirectory named include

Numerous problems with compilers and debuggers have meant that these freedoms are no longer recommended and all new packages should put everything in a single package (version) directory.