Underneath all this charming play of fancy, the fundamental plan of the movement is as clear as the outline of a mountain range under all the luxuriant foliage that clothes its slopes. This clearness of form is due chiefly to two causes, a fine logic in the use of themes, and a careful adjustment of keys. The closeness with which Mozart sticks to his thematic texts may in some cases at first sight escape us, but when we come to realize it through careful dissection, we cannot but be profoundly impressed by the intellectual grasp it indicates. Thus, the passage at measures 66-67 is not new, but is made from that of 48-49 _inverted_. The conclusion theme (72-88) is not made from new matter, as is usual with Haydn, but is derived from the little three-note motive of the first theme. The entire development is wrought out of new manipulations of the same theme, as is also the coda. The long transition in the recapitulation (191-225) is made entirely from the subsidiary. There is here, in a word, none of that "clattering of the dishes" between the courses. The economy of the master is everywhere observable; irrelevancies are excluded; there is no superfluity, no surplusage, no prolixity and wordiness. Every measure fulfils its purpose in the simplest and most direct way, and justifies its presence by its reference to the essential thematic ideas of the work.
Unity of key is secured by a careful observation of the main traditions of the sonata-form in the matter of the distribution of tonalities. The exposition shows the customary quality of key, tonic (G-minor) being contrasted with relative major (B-flat major).[30] The development, as we have already seen, exemplifies plurality of key. The recapitulation emphasizes throughout the home key of G-minor, thus ending the movement with the fitting impression of tonal unity. A glance at measures 38-42, 72-99, 134-138, 146-165, 221-225, and 260-307 will show how much pains Mozart has taken to emphasize his keys at all important points in the design. The emphasis, as in the case of Haydn, is superfluous for modern ears, but was very necessary for the audiences addressed by the early advocates of so complex a scheme of musical design.