Similar motion
Two voices move in the same direction.
What it means
Similar motion is the case in which two voices move in the same direction, that is, both upward or both downward. Unlike parallel motion, the distance between the voices does not stay the same: the intervals change, so the voices proceed in the same direction but not at the same distance.
Musical use
In part-writing similar motion is one of the four kinds of motion of two voices. It is considered less independent than contrary motion, because the voices follow each other in direction. Especially when perfect fifths or octaves are reached in similar motion, cautionary rules apply, since otherwise hidden fifths or octaves can arise, which are considered sonically weak.
Similar motion should be distinguished from parallel motion, in which the voices move in a constant interval, from contrary motion with opposite direction, and from oblique motion, in which one voice stays put. Parallel motion is a special case of similar motion.
Performance practice
When hearing and playing part music, note whether two voices move in the same direction and shape the connection so that each voice keeps its own line. Listen closely to intonation at passages where the voices arrive in similar motion at octaves or fifths.