CHAPEL COMPOSITION. - In the period of the greatest splendor of polyphonic music, when the most notable centers of musical life were the chapels and the most celebrated among composers were precisely singers or directors (masters) of them, the term "composizione a c." generically indicated all church compositions in polyphonic style, the performance of which was entrusted to choirs, not supported by any instrumental accompaniment.
The term continued to mean this even when (end of the 15th century) in some chapels the innovation was accepted of having the sacred offices accompanied by the sound of some instruments, and when, in the 16th and 17th centuries, the importance of organists in the chapels became of the first order (A. and G. Gabrieli in the chapel of St. Mark in Venice). In Germany, singing properly a c., much in vogue in the 15th-16th century, later diminished in importance, since the practice had greatly developed there of blending, in religious music, human voices ("solo" or "choir") with instrumental timbres, especially in the various forms of the so-called concerted style.
"Tempo a c." (or "alla breve") is the name given to that type of rhythmic division in which the measures or bars are made up of a breve, or four minims; it was much used in ancient church music, when the unit of measure was the minim, which seems to have ordinarily had a duration value similar to our quarter note. In modern usage the tempo a c. or alla breve can mean a 2/3 time (or even an ordinary 3/3 time beaten more quickly), marked to indicate the division of the bar into 2 minims; or it can still mean a 2/3 time marked C or also, but more rarely, C/O (from the ancient sign C/O by which the breve sometimes took on the value of the unit of measure, corresponding to 3 rather than 2 semibreves).
Anna Maria Gentili