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Extensio and fractio modi

In the previous video we gave you an overview of the basic technique of transcribing modal notation as well as of the fundamental principles of composition of the Notre Dame organum. Here you will find a short résumé of the two central compositional concepts, extensio modi and fractio modi.

Modal notation does not offer many possibilities for rhythmic variety. Its six patterns can of course be combined and the voices can change modes several times during the course of a composition. Nevertheless modal structure is somehow limited. The modal pattern of a musical phrase could be modified by interpolating single notes in the phrase. In order to bring more variety to the rigid modal structure, two compositional devices were employed, which were able to interrupt the uniformity of the rhythmical flow: the extensio modi and the fractio modi. In both cases, modal notation allows to make some adjustments to the rhythmic framework through the modification of the ligature structure.

The musicologist Willi Apel defines the extensio modi as follows:

‘This term is used here to denote the occasional omission of a brevis (eighth-note of the transcription). Such a modification of the modal pattern is very frequently used at the end of an ordo.’

Extensio modi from the Transcription of Clausula GoClausula Go. © Firenze, Biblioteca Medicea Laurenziana, Plut. 29.1 fol. 11r

This phrase is in the first mode. In the facsimile you can see a single note at the end of the ordo, which breaks the flow of ligatures. This note causes the interruption of the rhythmic pattern by extending the two last notes.

And here is Willi Apel’s definition of the second compositional practice, the fractio modi:

‘This term signifies the opposite procedure of extensio modi, that is, the breaking up of the normal pattern of the modal rhythm into smaller values, preferably of the longa into two breves.’

Fractio modi from the Transcription of Clausula GoClausula Go. © Firenze, Biblioteca Medicea Laurenziana, Plut. 29.1 fol. 11r

In the facsimile you can see a four-note ligature preceded by a single note at the end of the ordo. Since the phrase does not end with that note, it is not extended and it respects the pattern of the mode. The four-note ligature exceeds the mode by one note so that here at the very end of the phrase a fractio modi occurs. Instead of a long note value, two shorter will take place. A plica can also be considered a form of fractio modi, for at the end of the second ligature a smaller note value breaks the pattern of the mode.

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This article is from the free online course:

From Ink to Sound: Decoding Musical Manuscripts

University of Basel

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