equal temperament
A tuning system that divides the octave into twelve exactly equal semitones
In Depth
In equal temperament, each semitone has a frequency ratio of the twelfth root of 2 (approximately 1.05946). This makes every key equally usable but slightly out of tune compared to pure intervals derived from the harmonic series. Major thirds are noticeably wider than pure thirds, and perfect fifths are fractionally narrow. The system became standard for keyboard instruments by the mid-19th century and is now universal in Western music, though string players and singers naturally adjust toward purer intervals.
Bach's Well-Tempered Clavier was NOT written for equal temperament — it was written for a well temperament that made all keys playable but each with a different character.