temperament
A tuning system that adjusts the natural intervals between notes to allow playing in multiple keys.
In Depth
Pure or just intonation — tuning based on the natural overtone series — produces perfectly consonant intervals in one key but increasingly out-of-tune intervals as you move to distant keys. Temperament systems compromise individual interval purity to allow music in all keys.
Equal temperament divides the octave into twelve exactly equal semitones. This means no interval (except the octave) is acoustically perfect, but all keys are equally usable. The system was gradually adopted between the 17th and 19th centuries and is now standard for pianos and fretted instruments. Bach's Well-Tempered Clavier demonstrated that a well-tuned keyboard could play in all 24 major and minor keys.
Bach's Well-Tempered Clavier was a deliberate demonstration that all 24 keys could sound good — a revolutionary claim at a time when many keys were considered unusable on keyboards.