A Dictionary of Music and Musicians/Prolation
PROLATION (Lat. Prolatio; Ital. Prolazione). A subdivision of the rhythmic system, which, in Mediæval Music, governed the proportionate duration of the Semibreve and the Minim.
Prolation was of two kinds, the Greater, and the Lesser—called by early English writers, the More, and the Lesse, and by Italians, Prolazione Perfetta, and Imperfetta. In the former—usually indicated by a Circle, or Semicircle, with a Point of Perfection in its centre—the Semibreve was equal to three Minims. In the latter—distinguished by the same signs, without the Point it was equal to two. [See Point.] The signs, however, varied greatly at different periods. In the latter half of the 16th century, for instance, the Circle was constantly either used in connection with, or replaced by, the figure 3, to which circumstance we owe the presence of that figure in our own Time-Signatures, the Time now known as 3-2 being, in fact, the exact modern equivalent of the Greater Prolation, and that commonly called Alla Breve, 
, of the Lesser.
