Tempus
The mensural measure of the brevis.
What it means
Tempus designates, in mensural notation, the division of the brevis: it states whether it comprises two or three semibreves. It is thus one of several indications that together determine the mensuration of a piece — every level of the value hierarchy has its own name for the question of whether the division is duple or triple. The triple form is called tempus perfectum, the duple tempus imperfectum.
In the notation
Tempus is shown by the mensuration sign at the beginning: a full circle stands for the triple, a half circle for the duple division of the brevis. A dot inside the sign concerns the next-smaller level, that is, the division of the semibrevis. From the half circle the present-day sign for common time descended; it is thus not a C for “common time”, but the remainder of a circle that indicated the imperfection of the duple division.
Musical use
The distinction between perfect and imperfect has an ideological background: the triple division was regarded as the perfect one, because it pointed to the Trinity. In practice this ranking became less and less effective in the course of the fifteenth and sixteenth centuries, and the duple division prevailed — until, in the modern system, it became the sole rule and triple division had to be indicated specially, through dotting, triplets, or a triple time signature.
Tempus as an indication of division is to be distinguished from today's word tempo, which means speed. Both go back to the same Latin word but designate different things.
One distinguished tempus perfectum with three semibreves per brevis, indicated by a circle, and tempus imperfectum with two, indicated by a half circle. From this half circle arose today's time signature C.
Performance practice
When reading old sources, clarify the mensuration sign first, for without this indication the note values are ambiguous. For music-making, a modern transcription usually suffices; check the critical report to see how the editor resolved the mensuration, especially at changes within a movement.