Key-relationship is a modern idea, but it has its roots far back in musical history. The first traces of it are to be seen in the days previous to harmony, and under the ancient modal system. This system varied greatly from the modern scale, being at once more rich, and less flexible: yet it admitted some ideas of relationship between different tonalities, and those are analogous in many ways to modern key-relationship. The several modes, constituted as they are each differently one from another, naturally lent themselves to different kinds of modal relationship, some more to one kind and some more to another. Five of these kinds may be distinguished and briefly discussed, as they have an important bearing on the subsequent history of the art of music.