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When we put that (rest in peace) on a gravestone are we using an imperative voice?
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It may be that the original expression was 'May you/he/she rest in peace" in which case the answer would, I think, be no.

Some may argue that we are using an imperative chisel. Emotion: smile

edit .... Maven, it seems we wrote almost simultaneously!
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I had never given it much thought, but I guess it could be. Imperative is a mood, though, rather than a voice.

Or it could be an abbreviation of "(may s/he) rest in peace." (This would be the subjunctive mood, however.)
 Mike in Japan's reply was promoted to an answer.