十二平均律，又称十二等程律，是一种音乐的定律方法，将一个八度平均分成十二等份，每等分称为半音，是最主要的调音法。

公元400年左右，中国南朝数学家何承天提出世界历史上最早有记载的十二平均律数列 900 849 802 758 715 677 638 601 570 536 509.5 479 450（原文：……黄钟长九寸，太簇长八寸二厘，林钟长六寸一厘，应钟长四寸七分九厘强） .

意大利的物理学家伽利略·伽利莱的父亲伽利略·文森佐曾试图解决十二平均率问题，但他用的倍率是 18:17 而不是，因此自乘12次后只得 1.98556，不是2，八度走了音，他的系统只可算近似十二音阶平均律

An equal temperament is a musical temperament, or a system of tuning, in which every pair of adjacent pitches is separated by the same interval. In other words, the pitches of an equal temperament can be produced by repeating a generating interval. Equal intervals also means equal ratios between the frequencies of any adjacent pair, and, since pitch is perceived roughly as the logarithm of frequency, equal perceived "distance" from every note to its nearest neighbor.