Terzdezime

What it means

Terzdezime is an older term, handed down from nineteenth-century harmony treatises, for the thirteenth (13 steps, e.g. C to A). Contrary to first appearance, the name does not mean “third plus an octave” — that would be the tenth, a separate entry in the glossary.

Musical use

Compound intervals — those that reach beyond the octave — correspond in their function to their respective simple interval, but sound more open and transparent, because the notes lie farther apart. A third in a low, close position sounds muddy; the same third set as a tenth stays clear. For voice-leading this is a practical rule: in low registers, wide spacing is advisable.

In part-writing theory and in figured bass, compound intervals are as a rule reduced to their simple form — for the rules of voice leading, the tenth counts as a third, the twelfth as a fifth. From this it follows directly that parallel tenths are allowed and parallel twelfths forbidden.

In jazz chord theory, by contrast, extensions are expressly named as ninth, eleventh, and thirteenth, because their position above the seventh chord is decisive for the sound. There, the distinction between a sixth and a thirteenth is not a mere matter of form.

In the older thirteenth chord, the interval appears as a dissonance over a sustained bass and resolves stepwise downward, usually to the fifth or twelfth of the root; in this it resembles today's thirteenth, known in jazz as the extension tone 13 in dominant chords, whose sound seems full and lightly floating.

Performance practice

Grasp wide intervals beyond the octave by recognizing the simple interval and adding the octave, rather than counting through lines. Use wide spacing when a low texture sounds muddy. And with chord extensions, take care that the added notes lie on top, so the sound stays transparent.