Equal temperament
Tuning that divides the octave into twelve equal semitones.
What it means
Equal temperament is a tuning system that divides the octave into twelve equal semitones. Each semitone has the same frequency ratio, namely the twelfth root of two, about 1.0595, and measures 100 cents. All fifths are slightly narrowed compared with the pure fifth, all thirds somewhat widened compared with the pure third.
Musical use
The advantage of equal temperament is that all major and minor keys are equally playable and enharmonically equal notes coincide, such as C-sharp and D-flat. It has therefore become the standard system for the piano, organ, and many other keyboard instruments as well as for most modern ensembles. Until it became generally accepted in the 19th century, unequal temperaments were widespread, in which each key had a character of its own.
Equal temperament should be distinguished from just intonation, which tunes the intervals pure according to simple number ratios but does not allow all keys, and from the unequal, “well-tempered” systems of the Baroque. It is a compromise in which no key sounds perfect but all sound usable.
Performance practice
If you play a string or wind instrument, remember that you adopt equal temperament when playing with keyboard instruments, whereas in a string quartet the thirds are often tuned somewhat purer. Listen to the beats between the notes to judge whether an interval lies too high or too low.