The style basically involves four sections:
Basses who sing the notes on the lower line in the bass clef and whose function is to provide the driving force or the engine on which the whole song is sustained.
Lead section outlines the melody of the song which is of course the part most people are familiar with.
Tenors sing the notes on the top line of the treble clef often hitting an octave above the basses and frequently having to cross that bridge into falsetto voice. Without their voices we could not achieve the harmonic resonance and ring that can magically rise from a well executed chord.
Baritones sing the notes on the upper line of the bass clef although sometimes these notes drop below the bass voice. the effect on the overall sound is to enrich the depth and fullness of the harmony.
