Half circle (time signature)
Old time signature from which today's C sign for common time developed.
What it means
The half circle is an old time signature in the form of a semicircle open to the right, from which today's C sign for common time developed. It comes from the mensural notation of the Middle Ages and the Renaissance.
In the notation
In mensural notation the full circle indicated perfect, threefold division of note values, whereas the half circle indicated imperfect, twofold division. A vertical stroke through the half circle later marks the shortened alla breve measure. Today's “C” for four-four time is thus not a letter but a remnant of this sign.
Musical use
From the meaning “twofold” the idea of duple time developed over time. The crossed sign today stands for alla breve, two-half-note time. Whoever reads old editions often finds the half circle at the start of the line, where today the time signature stands.
The half circle should be distinguished from the full circle, which indicated threefold division, and from the modern fraction like 4/4, which expresses the same measure in numerals. The half circle is also not a sign for tempo but for the division of note values.
Performance practice
If you meet a half circle or a crossed half circle in an old edition, read it as common time or alla breve. In alla breve you count in half notes, so the music feels livelier and more flowing than in four-four.