Enharmonic
Notes that sound the same but are notated differently.
What it means
Enharmonic — in German also commonly spelled “engharmonisch” — designates two notes that sound the same on an instrument tuned in a temperament but are written differently and have different functions: C♯ and D♭, F♯ and G♭, B♯ and C. The equality is a consequence of equal temperament. In just or meantone tuning the two notes are not identical but differ by a comma, and on instruments with flexible intonation they can be distinguished.
In the notation
The spelling says which degree is meant and where the note goes next: a C♯ is a raised first degree and pushes upward, a D♭ a lowered second and pushes downward. That is why the choice of sign is not a matter of form. One speaks of enharmonic change when a note or chord is deliberately reinterpreted: it is reached in one meaning and left in the other, and in the score the spelling changes at this point.
Musical use
The most important use is modulation into remote keys. Especially suited to this are sonorities that are ambiguous in themselves: the diminished seventh chord consists of nothing but minor thirds and can, depending on the interpretation of one of its notes, be resolved in four different directions; the augmented triad divides the octave evenly and is similarly mobile. The reinterpretation often happens unnoticed — the ear follows the sound and suddenly finds itself in a remote key. In functional theory such a change is marked, because the same sonority means something different before and after the spot.
Performance practice
Read the spelling as an indication of direction: a sharp usually points upward, a flat downward, and the continuation confirms this. On instruments with flexible intonation, take the difference seriously — a G♯ as a leading tone lies higher than an A♭ as a lowered degree. With enharmonic reinterpretations, mark the spot when practising: there the harmonic context changes, even if the fingers notice nothing of it.