ff-related-file-alist

ff-related-file-alist is a variable defined in `find-file.el'.
Its value is

cc-other-file-alist

• Automatically becomes buffer-local when set.
• This variable is an alias for `ff-other-file-alist'.

Documentation:
Alist of extensions to find given the current file's extension.

This list should contain the most used extensions before the others,
since the search algorithm searches sequentially through each
directory specified in `ff-search-directories'. If a file is not found,
a new one is created with the first matching extension (`.cc' yields `.hh').
This alist should be set by the major mode.

You can customize this variable.