 This system recognized the double-long, long, breve, and semi-breve as the units of note value, related to one another by triple grouping; the double was always worth two longs, but a long could be perfect (and therefore worth three breves) or imperfect (and worth only two), depending on the exact sequence of notes.The breve was the "tempus", equivalent to the ‘unit of the beat’ in modern notation (three quarters to a measure in ¾ time, etc.