How does equal temperament make it possible to play music in every key?

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scale The universality and innate nature of music, and the pentatonic scale, as demonstrated by singer Bobby McFerrin.

Equal temperament makes it possible to play in every key by dividing the octave into 12 semitones of exactly equal size.1

Earlier tuning systems, such as just intonation, relied on acoustically pure intervals that accumulated intonational errors when transposed to keys with many sharps or flats. Because these systems were "cyclic," the intervals would change depending on the key, causing instruments with fixed intonation to sound out of tune in distant keys.1

In contrast, equal temperament distributes these minor intonational defects—such as narrowing the perfect fifth by a nearly imperceptible 2 cents—equally across all 12 tones of the chromatic scale. By standardizing the distance between every semitone, the system ensures that the relationships between notes remain consistent regardless of the starting pitch. This allows musicians to use enharmonic equivalence and modulate freely between any major or minor key without the instrument sounding out of tune.1