Oblique motion
One voice holds its pitch while the other moves.
What it means
Oblique motion denotes one of the basic ways two voices can relate to each other in polyphonic writing: one voice stays on its pitch while the other moves to a new one.
This distinguishes oblique motion from the two other basic types of motion, in which either both voices move in the same direction (parallel motion) or in opposite directions (contrary motion).
Musical use
In classical counterpoint and traditional voice-leading theory, oblique motion is regarded as a particularly unobtrusive, easily controlled type of motion, since one of the two voices, by holding still, provides a fixed reference point against which the other voice's movement stands out.
Oblique motion is often used deliberately to avoid or soften certain harmonic progressions, for instance to sidestep forbidden parallel fifths or octaves by having one of the two voices involved hold its pitch instead of moving along in parallel.
Oblique motion should be distinguished from parallel motion, in which both voices progress together in the same direction, and from contrary motion, in which both voices move simultaneously but in opposite directions.
Performance practice
When performing polyphonic writing, pay conscious attention to which voice holds still at a given moment, and keep that note sonically stable while the moving voice comes through clearly. This makes it easier to keep the individual voice leadings clearly audible in ensemble playing.