1959—Joseph Wagner, in his orchestration text, says the following about trombone

“Composers since the time of Gluck have, with few exceptions, consistently written for three trombones as a unit of ‘harmony instruments’…. Hector Berlioz, in his monumental Treatise on Instrumentation, referred to the trombones as ‘group instruments.’ This distinction results from their homogeneous tonal qualities, which vary little throughout their ranges. Any variance which may occur is generally the fault of the player’s execution of range extremes” (Wagner, Orchestration 226).