 Professional country music session musicians typically use music notated in the Nashville Number System, which indicates the chord progression using numbers (this enables bandleaders to change the key at a moment's notice).Chord charts using letter names, numbers, or Roman numerals (e.g., I–IV–V) are also widely used for notating music by blues, R&B, rock music and heavy metal musicians.Some chord charts do not provide any rhythmic information, but others use slashes to indicate beats of a bar and rhythm notation to indicate syncopated "hits" that the songwriter wants all of the band to play together.