What makes swing swing, and are the musicians late on purpose?

Swing is a technique, most commonly associated with jazz, in which the first and second notes of each two-part division of a beat are alternately lengthened and shortened. Instead of dividing the pulse into two equal halves, the pulse is divided unequally, so that eighth notes (or sixteenth notes) alternate between long and short.

How unequal? The first note is often understood to be twice as long as the second, giving a triplet feel. In practice the ratio is often much more subtle. In jazz the swing ratio typically lies between 1:1 and 3:1 and can vary considerably, and it tends to be wider at slower tempos and narrower at faster ones.

Is that the whole story? Not according to the musicians. Asked to define swing, Fats Waller replied, "Lady, if you gotta ask, you'll never know." Cootie Williams joked that he would rather tackle Einstein's theory. And physicists investigating swing have noted that it coincides with a perceptible difference between the timing of a soloist and the rest of the performers.

Are the musicians late on purpose? Some writers describe swing as deliberately playing behind or ahead of the beat. But one study of groove did not find an effect of microtiming, and a later one found that it decreased liking and the desire to move.