Equal temperament tuning

The octave is divided into 12 units of equal size.

All scales and chords of a kind are identical in interval relationships. There are no color-contrasts between the keys. Pitch is the only difference.
E.T. is the only suitable tuning for atonal or twelve tone music.
Many technicians feel equal temperament is the “only” tuning for the modern piano.
New research indicates true equal temperament was not practiced regularly until the early 20th century.