Thus we recognize the same system of associated lines in music as in architecture or drawing.

Very rarely indeed does one single unbroken line portray a complete image.
But in music, as in drawing, the lines differ in their degrees of importance and prominence; and, very commonly, one line over-shadows all, or nearly all the rest.

This strongest tone-line is therefore apt to be designated, somewhat unfairly, _the_ melody (the "tune" or "air" is more just).

But, at all events, _this predominating melodic line is the most important factor of the form, the one upon which the definition and recognition of the "form" depend_; and it is therefore necessary that the student learn to distinguish it, to acquire the habit of centring his attention upon it,--in reading, listening to, or analyzing music; and, in playing, to give it the emphasis it requires.
The importance of a tone-line depends solely upon its conspicuousness.
The principal melody--_the_ Melody--is the one which is most salient, which most attracts the hearer's attention.

For this reason the composer is induced to place his chief melody _above the rest of the tone-lines, because the uppermost tone strikes the ear more acutely than the lower ones_, and therefore the succession of highest tones constitutes a conspicuous line that attracts and impresses the sense most keenly.
Here then, at the top of the harmonic tone-complex, we look for the chief melody; and here it will be found,--excepting when arbitrary emphasis (by accentuation) is imparted to some lower tone-line, so that it, for the time being, assumes a prominence equal, or superior, to that of the uppermost line.