The math for tuning equal temperament (ET) wasn't worked out
correctly until 1911.
In a perfect ET, all semitones are exactly the same size. It contains no key-coloring and no tonality.
Equal temperament is:
Unrestrictive, because modulation through all keys is free from wolf intervals.
Regular, because all fifths are the same size.
Circulating, because it obeys the cycle of fifths.