Outer voices
The highest and lowest voice of a multi-part texture.
What it means
The outer voices are the highest and lowest voice of a multi-part texture – usually soprano and bass in four-part choral writing, or melody and bass in simpler settings. They form the harmonic and melodic frame within which the remaining, "inner" voices move.
Musical use
Traditional part-writing pays special attention to the outer voices because they are heard most clearly: the bass supplies the harmonic foundation and, together with the melody in the top voice, shapes the overall impression of a passage, while the inner voices mainly serve to complete the chords correctly. When working out a setting, the melody and bass are therefore often fixed first, with the inner voices filled in afterward.
A central rule of strict part-writing concerns the relationship between the outer voices themselves: parallel octaves and fifths between them are avoided, since they cancel out the voices' independence; contrary motion between melody and bass is likewise felt to be more satisfying than motion in the same direction.
Outer voices should be distinguished from inner voices, which lie between them and are led less prominently, and from the term "top voice," which refers only to the highest voice without including the bass.
Performance practice
When you analyze or write a multi-part setting, play or sing just the two outer voices together first: they will already tell you whether the harmony holds up and the voice leading is clean, before you turn to the inner voices.