MENSURABLE MUSIC (Ult. mensurabilis, measurable, from mcnsura, measure). Strictly speaking. all music written ill notes that have a definite time-value. In a specific sense the term is applied to the musie written between the be ginning of the twelfth and the seventeenth cen turies, before the invention of the line dividing a composition into bars. Before the twelfth century the choral note of the plain chant indi• eated only the pitch. The duration of each note was left to the individual singer, and arbitrarily determined by the rhythm of the text. As long as music was sung in unison this system an swered all practical purposes. But with the in troduction of harmony and the development of polyphonic music, employing a number of inde pendent voices, an imperative need made itself felt to fix the duration of the individual note. Alensnrable music, therm-fore, borrowed the forms of the notes as used in the plain chant. These were the large (maxima or duplex Ionga), the long (longa), 0,; the breve (br•ert's), 0: and the semibreve (semibrevis), *. To these were added the minim (minima), j, and semi.
minim (semi m in i ma). For For nearly three hundred years the notes were written in this form. At the beginning of the fifteenth century the black notes were gradually supplanted by the white or open notes : 0, For the smaller notes both the black and white forms continued in Vise : 43 or eroma or lima, or j; semieroma or seinifnsa, S , <3 or as earl' us the sixteenth cen tury rounded notes were substituted for the spuor ones in writing music. But it was not until 1700 that the round forms were generally adopted by music printers.
Out of reverence for the Trinity triple time was regarded as perfref time. whereas duple time was imprfect. A division of a note into three of the next smaller kind was mensuru perfrein; into two of the smaller kind, mensurn imprrfecto. This division WaS indicated by certain signs. hut a sharp distinction was made between the di vision of a long into breves. or of a breve into
semibreves. These signs were placed at (lie be ginning of a composition. The division of a large into longs or of a lung into breves was known as inothm; of a breve into semibreves as tempus; of a semibreve into minims to: protatio. The modes itself was further dislini.mished as //sodas major ( division of a large into longs) and modes minor (division of a long. halo breves). A still further subdivision of both the modus nml minor was into perfeetus (triple time) and imperjectus (duple). Tempos was thus also subdivided into perfcrtinn and imperfeetum; whereas in the ease of prolotio this division was designated as major and minor. The following table gives a complete view of this system with the various signatures: Alteratio was the doubling of the time-value of the second of two notes of the same kind when a. tripartite note of the next larger kind followed. Generally the two smaller notes stood between two of the larger kind, or were separated from the following notes of equal or smaller value by The sign of the moans major was the same for the perfect us and imperfcctus. The following sign for the mod as minor determined the modus major. If the sign was followed by III or III, it was modus major perfectas; if followed by II or I I , it was modus major impeifeet us. The length of the vertical bars had reference to the moth's minor. the long bars indicating the per fectus, the short ones the in perfectas. The num ber of vertical bars referred to the division of the large into two or three longs. The prelatic sign appeared only in connection with the tempos sign. Thus Q meant tempos perfect um, pro latio major ; C tempts impeilfeetum, prolatio major. If the tempos sign appeared without a dot it always meant that protatio minor was un derstood. The following table will make this clear: a punctilio divisionis. Thus in tempos perfect not (0, tripartite) p p'g"..•4 would be expressed in modern notation (values reduced one-half) „.