Three gaps, part 1

From David Eppstein’s blog I learned of a new Wikipedia article about the three-gap theorem. The who? I’d never heard of it myself, but was familiar with the general idea, having written about an application of it in a long article on scales for xenharmonic music.

In a more conventional music context, think about notes in the range from one note, say F, to the next F an octave higher. Scales, if you will. Here’s a representation of that. Ignore the black box for now and just focus on the white rectangle. That’s an octave, with our first note, F, at the left end.

(These diagrams may be rather small and hard to see; click on them to expand them.) Notes an octave apart are considered to be equivalent for our purposes, so you can think of the left end as being joined to the right.

One way to talk about the distances between musical pitches — intervals, as they’re called — is to use a unit called cents, where 1200 cents make an octave. We’re going to generate a scale based on an interval called a perfect fifth, which is X\equiv 1200\log_2 (3/2) cents — about 702 cents. So we’ll take our note F, shift it to the right by a distance X (see arrow below), and call that new note C.

We’ll call the interval between two consecutive notes a gap. Here there are two gaps, one from F to C which is a perfect fifth, 702 cents, and one from C to F which is a perfect fourth, 498 cents. These are shown in red and green respectively, and they add up to one octave, 1200 cents. This is a rather minimalist scale, two notes, with two kinds of gaps. (The number in the black box tells you how many notes are in the scale.)

Now go up a fifth again, starting from C. That puts you past the right end of the octave, but remember we identify the right end with the left end, so we can think of the arrow as going to the right end and then continuing from the left end to the new note, G.

G lands between F and C. It’s 2X above F, minus an octave, which works out to 204 cents, an interval called a whole tone. From G to C is 702 cents minus 204 cents or 498 cents, so G splits the perfect fifth from F to C into a whole tone (in blue) plus another perfect fourth (again in green). In this three note scale there again are two kinds of gaps.

Going up another perfect fifth gives us D, between C and F and splitting that perfect fourth into a whole tone and a major third (294 cents). Now we have a four note scale with three kinds of gaps. The major third is orange.Notice the big gap (perfect fourth) is equal to the sum of the other two.

The next perfect fifth gives us A, which splits the remaining perfect fourth into a whole tone and a major third. This five note scale is back to having two kinds of gaps.

Add a sixth note, E,  and one of the major thirds gets split into a whole tone and a diatonic semitone (90 cents), shown here in purple. There are three kinds of gaps. The big one (major third this time) is again equal to the sum of the other two.

But adding a seventh note, B, splits the other major third into a whole tone and a diatonic semitone, and once again there are two kinds of gaps.

At this point we have a diatonic scale, or in less pedantic terms, a C major scale if you start on C. (Or A minor if you start on A.)

You start to see a pattern. Each scale has at most three kinds of gaps. If there are three kinds, then the largest is equal to the sum of the two smaller ones. When you add a note to such a scale, it splits one of the large gaps into one of each of the two other kinds; eventually you split all the large gaps and are left with a scale having only two kinds of gaps. But maybe we’re getting ahead of ourselves? That’s the pattern so far, but does it continue?

We’ll see in part 2.


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