I'm trying a new weekly feature, answering interesting questions that show up in my referrals.
Today's question: What is a ditone chord? This question led to my post on the Giant Steps analysis by Haughton and Flanagan. A ditone can be generally defined as a major third (two whole steps), or more specifically as the Pythagorean tuning for a major third, the ratio 64:81, as opposed to the just tempered tuning of 4:5. But this is an interval, not a chord. The term "ditone chord" is used in gospel music to describe a chord based on the third scale degree. In the key of C major, some kind of E major chord is the ditone chord. In classical music theory, this would be described as a III chord or a chromatic mediant chord. But in gospel this is treated as a variant of the tritone substitution, as a way of adding chromatic tension to the tonic chord. Tritone substitutions are used by jazzers to liven up a ii-V-I progression with a chromatically descending bass line and extra implied extensions and alterations to the original V chord (it is essentially a V#11 b9 with the #11 in the bass). I imagine the ditone chord is used in a similar way in gospel, but I'm not so familiar with that genre.