Until the Osbornes' 1958 hit "Once More", the typical arrangement called for a "lead" singer to provide the melody with a tenor singing a higher part, and a baritone below.
Prior to the Osbornes' 1958 chart-topper "Once More", the standard vocal configuration called for a "lead" singer to deliver the main melody, with a tenor adding an upper harmony and a baritone providing a lower harmony part.