Mediant
In the context of just intonation ratios, the mediant M of two ratios a/c and b/d in lowest terms is M=(a+b)/(c+d). It will always be between the two ratios (a/c < M < b/d, assuming a/c < b/d).
Examples
The following table shows the mediant m of some fraction pairs f1, f2.
f1 | f2 | m | intermediate step(s) |
---|---|---|---|
3/2 | 5/4 | 4/3 | (3+5)/(2+4) = 8/6 |
3/2 | 4/3 | 7/5 | (3+4)/(2+3) |
5/4 | 6/5 | 11/9 | (5+6)/(4+5) |
9/8 | 10/9 | 19/17 | (9+10)/(8+9) |
9/8 | 19/17 | 28/25 | (9+19)/(8+17) |
19/17 | 10/9 | 29/26 | (19+10)/(17+9) |
Generalization
The mediant operation can also be used to find generators and scales in edos representing temperaments. For example, the perfect fifth (3/2) in 12edo which supports meantone is 7 steps out of 12, and the fifth in 19edo, another meantone tuning, is 11 steps out of 19. Hence the perfect fifth in 31edo (which is a meantone tuning because 31 = 12 + 19; more precisely, the 5-limit val of 31edo is the sum of the 5-limit vals of 12edo and 19edo) is (7+11)\(12+19) = 18\31, which is in between the sizes of the 12edo fifth and the 19edo one.