Chiavette
High clef combination of the Renaissance.
What it means
Chiavette — literally “little clefs” — designates in the music of the sixteenth and seventeenth centuries a clef combination in which all the voices of a piece are set higher than under the usual clefing. Instead of the usual combination, higher clefs appear throughout, for instance the treble clef for the upper voice and the tenor clef instead of the bass clef for the lowest voice. The term itself is younger than the thing; it does not appear in the sources in this form.
In the notation
The phenomenon can be recognized only by comparison: it is not a single unusual clef that constitutes it but the clefing of all the voices of a piece shifted upward throughout. A particular key signature often goes along with it. Since in the vocal writing of that time every voice carried its own C or F clef anyway, the deviation easily escapes today's reader if they do not pay attention to the overall picture.
Musical use
What this way of writing amounted to is still disputed in scholarship. One widespread interpretation sees in it an indication of transposition: the piece is to be performed not at the notated pitch but lower, often by a fourth or fifth. Another explanation understands the choice of clef simply as a means of getting the range of the voices onto the staff without ledger lines, without any change of sound being meant. Both views have source arguments on their side, and practice was probably different according to place, time, and genre.
The chiavette question is to be distinguished from ordinary transposition: there a transposition is expressly requested or notated, whereas here it is disputed whether the notation means one at all.
Performance practice
When you rehearse music of this period with an ensemble, follow the decision of your edition and read the editor's report instead of transposing yourself — the choice of pitch concerns the entire piece and must be the same for all voices. Instead, test it in practice: if the notated pitch lies uncomfortably high for the voices, that is an argument for a lower performance. When in doubt, decide by what the singers can carry sonically, and fix the chosen pitch before the first rehearsal.