同主音调，常被称为同主音大小调。它指的是两个调的主音相同，但音列不同的关系。

例如C大调的同主音小调是c小调。

一般地，两个同主音大小调的调号不同。例如C大调的调号不包含任何升降号，而c小调的调号由三个降号组成。

和同主音调相对应的是关系调。由于中国近代乐理主要有俄文翻译而来，所以平行大小调只的是关系大小调，而非同主音大小调。

In music, a major scale and a minor scale that have the same tonic are called parallel keys and are said to be in a parallel relationship. The parallel minor or tonic minor of a particular major key is the minor key based on the same tonic; similarly the parallel major has the same tonic as the minor key. For example, G major and G minor have different modes but both have the same tonic, G; so we say that G minor is the parallel minor of G major. In contrast, a major scale and a minor scale that have the same key signature (and therefore different tonics) are called relative keys.