Here let us observe the value of key relationships.  The first subject was given out in the key (say) of C. A momentary pause was made, and the second subject introduced in the dominant key G, and in this key the first section of a piece of music in symphony-form ends.  That ending could not satisfy the ear, which demanded something more in the first key.  Until recent times that desire was gratified with a repetition of the whole first section.  The repetition of the first theme in the first key satisfied the ear for the moment, though at the end of the section the want was again felt.  So when the end of the first section was again reached a modulation was made, gradually or suddenly, to another key; and in the course of this, the development or “working-out” section, many keys might be touched on, but without ever giving the ear the satisfaction of feeling itself at rest in the first key again.  That was only done by the reintroduction of the first theme in the first key.  The first theme is played and leads on to the pause, after which the second theme is given in the key of the first, so that after a few bars of coda, always in the same key, the movement terminates in a perfectly satisfactory manner.  This is a crude description in which much is left out, but it will serve to enable the reader to understand how passages widely different in character are bound together into a coherent whole by the composer continuously leading the ear to expect something—­that something being the original key-chord, and, while offering many things, only finally satisfying the ear’s craving when the movement is coming to a finish.  If the second theme, let us say, were in the same key as the first, it would sound like the beginning of a new movement, and at once we should have the continuity broken.  As a passage between two passages in the original key it sounds perfectly in its place, and, no matter how contrasted in character, is a kind of continuation of the first passage.  At the same time it creates a strong desire, that must be restrained till the time comes, for what follows.  We listen to the second theme and to the “working-out” section, knowing we are far from home, but perfectly aware that we shall get there, and that a certain feeling of suspense will be relieved.  Thus the music is like a great arch that supports itself.  The unity got in the fugue by continuous motion is got here by one key perpetually leading the ear to ask for another key.  It seems simplicity itself; its underlying idea—­that of making the ear always expect something, and gratifying it by bits, and only fully towards the close of the movement—­is that by which unity is combined with variety in modern music, though we have long since got rid of the “legitimate” series of keys.