Equale

An equale or aequale (from , equal voices or parts) is a musical idiom. It is a piece for equal voices or instruments. In the 18th century the equale became established as a generic term for short, chordal pieces for trombone choirs, usually quartets or trios. The instruments were not necessarily equal in pitch, but formed a closed consort.
Commemoration of the dead.
Aequales were conventionally used in Austria to commemorate the dead. They were performed from towers on All Souls' Day (2 November), and on the previous evening. They were also performed at funerals.
While aequales might be played by other instruments, the sound of trombones was thought to be especially solemn and noble. Trombones had also already acquired an association with death and the afterlife. Finally, the theological symbolism of the trombone, representing divine presence, the voice of the angels, and the instrument of judgment, was thereby underscored.
Examples.
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. Two of them were later performed, with the addition by Ignaz von Seyfried of words from the "Miserere", at Beethoven's own funeral in 1827. They were also played as instrumental pieces at the funeral of William Gladstone in Westminster Abbey in 1898. 
The two "Aequali" in C minor of Anton Bruckner date from 1847 and are for three trombones. Three years earlier, in 1844, the little-known Wenzel Lambel (1788–1861) of Linz had published ten equali for three or four trombones. 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.