A system of notation established around 1260, remaining in use until about 1600. Initially, the three principal note values in use were the long, breve, and semibreve. The long was equal to three breves, and the breve equal to three semibreves. An additional fourth note, duplex long, was equal to two longs. By the 14th century, the minim was added and by the 15th century, the semiminima and fusa were also added. In Mensural Notation, several notes can be combined together to form ligatures. In a ligature, the relationship between any two adjacent notes can be either triple (3) or duple (2). The relationship between the long and breve is the modus, and if the modus is triple (three breves to the long), then it is major. A duple modus (two breves to the long), is said to be minor. Similarly, the relationship between the breve and semibreve is the tempus, and if the tempus is triple (three semibreves to the breve), then it is perfect. A duple tempus (two semibreves to the breve), is said to be imperfect. Finally, the relationship between the semibreve and minim is the prolatio, and if the prolatio is triple (three minims to the semibreve), then it is major. A duple prolatio (two minims to the semibreve), is said to be minor.Also see notation. a system of rhythmic notation (late thirteenth century - ca. 1600) in which each shape stands for a particular rhythmic value. The commonly used noteshapes were the long, breve, semibreve and (by the fourteenth century) minim. Each value could be worth either two or three of the next value down and was called imperfect or perfect, respectively. Mensural music is distinct from music employing the rhythmic modes in which the context-dependent notation limited rhythmic flexibility, and from unmeasured music in which no set rhythmic values were employed.