' is to chord files
' is to tablature
'crd' is the file extension given to plain ascii textfiles which contain instructions for playing songs on guitar.
'CRD' is also used in subject headers in the alt.guitar and rec.music.makers.guitar usenet newsgroups to show that the body of that particular article contains the chords to a song. For example, 'Subject: CRD U2: Stuck in the Moment'.