One thing not meantioned for bluegrass is the limitation of instruments in the genre. Stringed instruments only and acoustic, sometimes amplified with a microphone. Further, the banjo is only 5 string, not 4 string tenor or plectrum. The mandolin often takes a percussion role through the use of "Chop Chords". Other allowed instruments are: fiddle, stand up bass, guitar.

It's not so definitive. One of Bill Monroe's "Blue Grass Boys," for some years, was a woman named Wilene "Sally Ann" Forrester, who played the accordion. At the height of their popularity, harmonica player Charlie McCoy was a member of Flatt & Scruggs' Foggy Mountain Boy.