I guess it's some type of trombone, but I can't find its correct name anywhere.
P.S. If you're wondering, here's where I got this shot from:
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Sign up to join this communityThis is apparently called a contrabass valve trombone, or sometimes a cimbasso if I am correct.
This is a cimbasso (a valve contrabass trombone). The instrument is mainly used in Verdi's operas (and one opera each by Bellini and Puccini). The modern instrument has six rotary valves and is normally pitched in F, but there are also instruments in B-flat (a fifth lower). Both instruments can be heard here (the low B-flat is the one with the smaller bell, lower right):
The instrument shown is called a cimbasso, an instrument basically created by Verdi as something similar to the contrabass trombone. It's pretty rare!