The Latin word "temperatio" means proportionality, the right balance. Educated from a musical term refers to the alignment of the correlations between the degrees of the scale, the establishment of a system of sounds of different heights.
Now all pianos have an even temperament: each octave is divided them into 12 equal intervals - tones. It was not always so. In the Middle ages there was other music system. They temperament has been uneven, and in different keys with the same names intervals sounded different. Therefore, when was invented the ancestor of the modern piano, the first tool with an even temperament, admiring them Johann Sebastian Bach wrote his famous cycle of preludes and fugues in all twenty-four keys and called it "Well-tempered clavier".