Voice crossing
Voices exchange their registers.
What it means
One speaks of voice crossing when a voice moves above or below another that, in the ordering of the texture, actually lies higher or lower — the alto rises above the soprano, the tenor below the bass. For a moment the voices exchange their registers, without giving up their identity as voices.
In the notation
In a score with separate staves, the crossing is easily overlooked, because every voice stays on its own staff and only the pitches overlap. If two voices share one staff, stem direction gives information: the upper voice keeps stems up, the lower one down, even when their notes are swapped. Precisely for this reason this rule is more important than the spatial proximity of the noteheads — with crossings, the eye is otherwise misled.
Musical use
There are several reasons for a crossing. It can result from the voice leading, if a line consistently continues its path. It can avoid a forbidden parallel. And it can be tonally intended: a lower voice temporarily lying on top changes the colour of the texture. In vocal polyphony it is used sparingly, because it makes clarity of hearing more difficult; in piano writing and in nineteenth- and twentieth-century music it appears more often and occasionally becomes a means of shaping.
Voice crossing is to be distinguished from voice exchange, a medieval technique in which two voices swap their material with each other, and from contrary motion, from which a crossing can arise but which is not itself one. In strict writing it counts as permitted but should remain brief.
Performance practice
In polyphonic textures, watch the stem direction before deciding what belongs together — with crossings it is the only reliable information. Practise the voices separately and, when joining them, deliberately continue each one, so that the line stays audible across the crossing. Bach places it often in chorales between alto and tenor.