The triangle was a small, handheld, percussive musical instrument that the Eleventh Doctor claimed to have played in a recording of "L'amour est un oiseau rebelle" from the opera Carmen. He said that he couldn't be specifically heard because he "got buried in the mix". (TV: Asylum of the Daleks)
Iris Wildthyme played the triangle on David Bowie's Low/Profile in 1977. (PROSE: Low/Profile [+]Loading...["Low/Profile (short story)"])