diesis

Diesis is a historical term in music theory used to denote a small interval, typically described as a chromatic semitone or a sharpened pitch, found in ancient Greek theory and carried into medieval writings. The concept was used to explain how scales and tunings could be divided into smaller steps beyond the basic diatonic semitone.

In Greek theory, diesis contrasted with limma, the diatonic semitone, and was involved in the discussion of the enharmonic and other genera of the tetrachord. The diesis helped describe how a note could be raised to form a chromatic pitch within the system, contributing to the understanding of pitch relationships before the widespread adoption of equal temperament.

During medieval and Renaissance theorizing, the diesis remained part of various tuning and genus discussions, often in connection with the idea of sharpening or altering notes to create chromatic steps. Its exact size depended on the tuning or temperament being described; as tuning practices evolved, the practical use of diesis diminished.

In modern musicology, diesis is primarily of historical interest. The term is sometimes used in discussions of microtonal theory to refer to a chromatic semitone, but contemporary discourse more often describes such intervals in terms of specific ratios or cents rather than the older label diesis. Related concepts include the limma (the diatonic semitone), chromatic semi-tones in different temperaments, and the broader study of meantone and other historical tunings.

Diesis thus represents an historical approach to small pitch steps that informed later developments in temperaments and intervallic theory, even as the terminology fell out of common use.