It is hard to make those unacquainted with musical technique understand how much of fixity there is in the musical idiom, how definite are the types of musical form, how potent the requisitions of musical syntax. Yet, without a sense of this fixity in the material, it is impossible to estimate justly those impulses and motives which may lead a composer to violate usages and to disappoint expectations. In the matter of harmony, for instance, there are certain types of procedure, certain progressions and sequences of chords, that are as stable and uniform as the types of animal or vegetable form. A horse, a dog, or a man is not a more definite organism than the two chords in the «Amen» of a hymn tune. This group or cluster of two chords, linked together by a common tone held over from one to the other, yet made distinct by progression of the other voices, is typical of a kind of harmonic form that long usage has established as part of our mental furniture. We are used to thinking of chords thus welded by a common tone, and we demand this sort of coherence in our harmonic progressions, just as we demand that a horse's body shall be furnished with a horse's legs, or that a transitive verb shall have an object. To be sure, this particular sort of cluster, in which both chords are, as we say, consonant, is somewhat less determinate than another sort which we shall describe presently, because, since all the tones of the first chord are equally important, any one may be selected as the link, and there will be consequently some latitude in the choice of the second chord, which completes the group. But within these limits this sort of harmonic type is definite and fixed, and that it is deeply ingrained in our mode of thought is proved by our horror of «consecutive octaves» and «fifths,» those bugbears of harmony students, which are bad chiefly because they are not compatible with the retention of a common linking tone between the two members of the group.