7

This number is a prime.

In "Western" music's 12-tone scale, ratios of fractions between 2 and 1 with a one-digit denominator that involve 7 are not used. This is true whether 7 is either in the numerator or denominator. Ratios such as 7/4, 7/5, 7/6, 8/7, 9/7, etc. are often common notes in the music of other cultures or blues music, but not the "Western" equal temperament scale. [Obeidin]