The four motions

Two voices can relate in only four ways: parallel motion (same direction, interval kept), similar motion (same direction, interval changing), oblique motion (one holds, one moves), and contrary motion (opposite directions). The grammar of two-voice conduct follows in three rules: a perfect consonance is reached only by contrary or oblique motion; an imperfect consonance may be reached by any motion; and the richer the mix of motions, the more independent the voices sound — with contrary motion the surest guarantor of independence.

It is the first vocabulary of part-writing — every rule about fifths, octaves and independence is stated in terms of these four words, so they come before everything else. In practice the mix is the message: a passage that leans on parallel and similar motion binds its voices together, one that leans on contrary and oblique motion pulls them apart, and a composer steers the sense of independence by rationing the four.

• Reaching a perfect fifth or octave in similar or parallel motion, the combination the three rules exist to forbid.
• Writing long stretches of one motion — all parallel, or all contrary — so the texture turns mechanical either way.
• Forgetting that oblique motion is available: a held tone is often the cleanest legal route into a perfect interval.
• Treating the rules as applying only to adjacent voices, when every pair in the texture is bound by them.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, the four motions and the three rules — The opening doctrine of the dialogue: the motions defined, then the three rules of approach.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules of counterpoint — The Conservatoire restatement that carried the same grammar into the nineteenth century.