[1][2] In the 18th century the equale became established as a generic term for short, chordal pieces for trombone choirs, usually quartets or trios.
[3] Finally, the theological symbolism of the trombone, representing divine presence, the voice of the angels, and the instrument of judgment, was thereby underscored.
[4] Notable examples of the genre are the three Equali for four trombones of Ludwig van Beethoven ("Drei Equale", WoO 30, see score), written for Franz Xaver Glöggl and performed in Linz Cathedral on All Souls' Day (2 November), 1812.
Three years earlier, in 1844, the little-known Wenzel Lambel (1788–1861) of Linz had published ten equali for three or four trombones.
[6] Stravinsky scored In memoriam Dylan Thomas, his setting of "Do not go gentle into that good night", for tenor, string quartet and four trombones, which may be an "echo" of the tradition.