This is not a guillotine.

A list I subscribe to recently had a small item about Francis Poulenc's Dialogues of the Carmelites (1957), which is not exactly the kind of thing I write about here.  Dialogues may well be the most-performed opera of the second half of the 20th century.  It's a lush piece, requires limited stage resources, has forgiving vocal writing and is not overly long.  But it's not really a work which pushes any boundaries.  

But this item did mention an interesting feature in the work: two players from the percussion section are assigned the responsibility for the sound of the (off-stage) guillotine, "one producing a crescendo-glissando with a metal object on the edge of a tam-tam(laid flat in order to create a dry scraping sound) as a grace-note to a hammer-stroke on a large wooden box produced by the other player."

This happens to be a superb illustration of musical representation:  Poulenc, writing in 1957, could well have used a recording of real guillotine, or even positioned a real guillotine offstage and have it slice through something head-like (a ham, or perhaps a melon?) for each decapitation in the score, that is to say, treated it like a film or stage sound effect (in film, this would probably be assigned to a sound effects artist, possibly a foley artist, although, not being produced by visible hands or feet, it is not, technically, a foley effect).  But Poulenc decided to treat it as a musical representation, timed to the score, produced by percussionists in the pit, and with not only a technical description of the sound production but a precise musical description of the sounds required and their placement in musical time.  

This sound in Dialogues is thus something that the listener will immediate identify within the narrative as a guillotine, but it is also more than that as the noises are tightly integrated into a rhythmic and tonal context.  It is even possible to imagine the same striking sound occurring in different works of music, completely divorced from its representative function. This is, of course, typical of the history of the European orchestral franchise as, for example, horns and trumpets and trombones gradually became members of the ensemble, initially representing functional sounds, associated, respectively, with the hunt, battle, and the church tower, yet eventually becoming integrated into the ensemble for more strictly — and abstractly — musical functions.   Aside from occasional visits by instruments like the saxophone (an instrument carrying its own associations), the last two centuries in the history of orchestration have featured the gradual addition of percussion instruments, often introduced first in the opera, typically first through either such literal representations of diegetic sounds or the invocation of the exotic.