While this is happening in the right hand, the left hand is doing the same but in reverse, starting with the semibreve and working towards the semiquavers.
The smaller unit could also be replaced by a group of yet smaller notes of equivalent length, e.g., a semibreve's worth of minims or semiminims (ex. d).
The relation between the breve and the semibreve was called tempus, and the relation between the semibreve and the minim was called prolatio.
A whole note (semibreve) divides into two half notes, a half note into two quarters, etc. and other notes are made by tying these together.
The difficulty was compounded in the later half of the 13th century, when the lozenge shape came also to be used for the semibreve.
I was particularly pleased to see a serious discussion of the possibility of semibreve equivalence between signs, for there are musical sources where this provides the simplest explanation.
Moreover, the occasional odd extra semibreve in all parts is accepted editorially into a bar so that cadences always fall at the beginning of a bar.
In his twelfth rondeau is found an example of a brevis divided into four semibreves.
