One clear, predominating tone-line traces the "air" or tune of the piece; and this is often the only line that arrests the hearer's attention; but there are other tone-lines, less prominent and less extended and coherent, gliding along harmoniously beside the Melody proper, which (something like the shading in a picture) contribute to the richness of the design, and perform their share in proving and illuminating the Form of the whole.
This is most salient in music for orchestra, where each player describes an individual tone-line, rendered all the more distinct and recognizable by the specific "color" of his instrument; and that is the chief, perhaps the sole, reason why the orchestra is esteemed the most complete and perfect medium of musical expression.
UNITY AND VARIETY .-- As much as opinions and beliefs may differ, among music critics, as to the necessity of Form in music, and the conditions of its existence, no reasonable objection can be taken to the hypothesis that _Clearness and Attractiveness_ are the two vital requisites upon which the enjoyment of any art depends.

The artist's utterances or creations must be intelligible, and they must be interesting.

The lack, partial or total, of either of these qualities neutralizes the force of the intended impression, in precise proportion to the default.
In musical composition these two requisites are embodied in the principles of Unity and Variety.
_Unity_--in its various technical phases of Uniformity, Regularity, Similarity, Equality, Agreement, or whatever other synonym we may find it convenient to use--is the condition out of which the composer must secure intelligibility, clearness, definiteness of expression.

Glance at Ex.

2, and note the evidences of unity (similarity) in the rhythmic and melodic formation of the first four measures.
_Variety_--in its most comprehensive application--is the medium he must employ to arouse and sustain the hearer's interest.