ONE MUSICIAN is singly indispensable. One figure on stage, one voice on the track, one name on the sleeve and the marquee, one face (dramatically lit) on the album cover.

A DUO is a variation on a marriage; sometimes it is a marriage. Two forces set against each other—a planet and its moon, yin and yang, an egg and its shell. You are forever an outsider to its workings, whether it’s a pair of twins or binary stars. It is beautiful but terrifying.

A TRIO has a severity to it. Getting it right achieves some law of physics but it often buckles under the strain. Think of three elephants on bicycles, wheeling in loops around each other. Or The Police.

A QUINTET is a singleton and a backing band, or a couple in confederacy against the rest. What matters is who has the controlling shares.

A SEXTET, when it works, which is rarely, is ungainly.

A SEPTET and beyond has a collective beauty but the more members it has, the more it calls up the sprawling family table—faction within faction, an overgrown cousinage. At some point, communism or tyranny are the only real solutions.

Then there is THE QUARTET. An even-cornered set. One rules three. Two pairs date each other. Two sisters have two friends. Everyone only gets along with the drummer.

It always has the potential of order. Four pegs make a tent. Foursquare. Four to the bar. Trading fours. The four seasons, humors, card suites, phases of the moon, elements, gospels, points of the compass. First violin, second violin, viola, cello. Soprano, alto, tenor, bass. Lead guitar, rhythm guitar, bass, drums. One-two-three FOUR!

This is a study of 64 QUARTETS. They are not the 64 best quartets. They are not all my favorites, nor likely yours. The rules for inclusion in this series are precise and arbitrary.