Semibrevis
The mensural value “half brevis”.
What it means
The semibrevis is a note value of mensural notation. Its name means “half brevis,” and indeed it corresponds to today's whole note — the brevis, in turn, to the double whole note.
In the mensural system the ratio of the values was not consistently twofold. Whether a brevis contained two or three semibreves, and whether a semibrevis divided into two or three minims, was fixed by the mensuration signs at the beginning of the piece. Today's rule, that every note value is exactly double the next smaller one, became established only over time.
The modern designation of the whole note in several languages goes back to this value; in English it is still called semibreve today.
In the notation
In old sources, the semibrevis appears as a diamond-shaped note without a stem, at first blackened, later as a white diamond. In modern transcriptions it is rendered by the whole note, often with all values halved, so that the semibrevis becomes a half note. Which procedure an edition chooses stands in the preface — a look there is necessary before determining tempo and counting.
Musical use
In the vocal polyphony of the fifteenth and sixteenth centuries, the semibrevis is one of the basic values and, together with the brevis, carries the notation. The large values correspond to a slower counting, not to slower music: what would be notated today in quarters and eighths stands there in semibreves and minims. Whoever reads faithful editions must factor in this shift.
From the semibrevis, today's whole note developed. In mensural notation it had the shape of a diamond without a stem.
Performance practice
Check first whether the edition has transcribed the values; how you count depends on this. With faithful notation, think in larger units, so that the notation does not seem cumbersome. Count long values out fully and plan breath or bow accordingly. And with vocal music, take your bearings from the declamation of the text when choosing the tempo.