TonalityGUIDE - Tonal music basic theory for Undergraduates
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The church modes date back to at least the Eighth Century and, although their relationship to ancient Greek modes is uncertain to say the least, they were nevertheless given Greek names in the Ninth Century.

In the tonal system accidentals are used to allow the transposition of the major and minor modes to begin on any note of the chromatic scale. The sequence of tones and semitones thus remains the same relative to note on which the scale is based. The modal system, however, presupposes that the tones and semitones are fixed between E-F and B-C and modes are distinguished by the note upon which they end (the finalis). The modes have a note on which phrases other the final one may also end, this is called the tenor (circled notes below).

You may notice that the Ionian and Aeolian modes correspond to our major and minor scales - they were proposed in the Sixteenth century as theory caught up with practice! The Locrian mode, starting on B is included for completeness. There is little evidence that it was used in practice.

A major difference from the modern tonal system is that closure in modal writing is primarily defined melodically rather than harmonically. The perfect cadence is the standard way of ending a tonal piece of music. In modal music, final closure was defined by a particular voice-leading pattern as much as by a succession of chords. One of the most common pattern in a two-part texture was for the upper voice to rise and the lower voice to fall onto the final of the mode that the piece was in, as shown in the examples below. The tonal cadences at the end of each example could be said to derive from these earlier closing gestures.