Volti subito
Turn the page quickly, an instruction in the notated text.
What it means
Volti subito means “turn at once” and stands at the bottom edge of a music page when playing continues immediately after the last bar and no time remains for turning the page. The instruction is not addressed to musical action but to a purely practical matter — and belongs precisely for this reason among the points where something most often goes wrong in ensemble playing.
In the notation
Usual is the three-letter abbreviation, at the bottom right of the page, occasionally additionally with an arrow. In well-laid-out editions it is rarely needed, because the setting is arranged so that page turns fall on rests; where that is not possible, the notice appears. With four-hand editions, in which both players look at the same double page, and with chamber-music parts of dense texture, this cannot always be avoided.
Musical use
The necessity of turning pages has always influenced the layout of music editions: the distribution of the bars across the pages is a balance between clarity and turnability. Historically, the custos sign at the end of a line helped keep one's bearings at the change. In orchestral parts, two string players share one stand, so that the outer player can turn while the inner one keeps playing — a solution that organizationally defuses the problem.
The abbreviation reads V.S. and stands at the bottom right edge of the page. It warns that the music continues without a pause and that the page turn must happen quickly.
Performance practice
When working on a piece, check all page turns before you play it through for the first time, and treat them as passages to be practised. Where things get tight, there are several ways out: paste a copy of the first bars of the following page onto the margin, have the pages laid out differently, play across the turn from memory, or clarify the turning role within the ensemble. Turn early and with whichever hand happens to be free, rather than at the last moment. And practise the turn with the instrument, not only in your mind — the movement must sit like a fingering.