Posted by Jane on 18/4/2021, 18:37:33, in reply to "Photo on top"
It's a collective style, "Their Serene Highnesses", just as TRH stands for "Their Royal Highnesses" (if one were referring to, say, the sovereign grand ducal couple of Luxembourg).
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I've been wondering what TST stands for --their serene toddlers? It's a cute photo, to be sure.