The best jazz singers use their voices much as horn players use their instruments. The great Billie Holiday thought of her singing voice as a horn, reshaping melody and words to increase their impact. Conversely, jazz horn players achieve their distinctive sounds by emulating the spontaneous twists and turns of an impassioned voice. So jazz consists largely of voicelike horns and hornlike voices.
The claim that the best jazz singers use their voices much as horn players use their instruments is a statement used to support a conclusion that in turn is used to support the argument's main conclusion.