Semicomma

2109375/2097152 = [-21 3 7⟩, called the semicomma or Fokker comma, is an interval of 10.061 cents which is the amount by which three classic augmented second intervals of 75/64 exceed 8/5. That means (75/64)^3/(8/5), which can also be written ((9/8)*(25/24))^3/(8/5). Tempering it out leads to orson temperament.