Rules of Victorian Mystery Novels

"[Victorian mystery novels] always take place in a country house...and the butler did it, at least for the first hundred mystery novels or so. Everyone's a suspect, and it's always the least likely person, and after the first hundred or so, the butler wasn't anymore--the least likely person, I mean--so they had to switch to unlikely criminals. You know, the harmless old lady or the vicar's devoted wife, that sort of thing, but it didn't take the reader long to catch on to that, and they had to resort to having the detective be the murderer, and the narrator, even though that had already been done in The Moonstone. The hero did it, only he didn't know it. He was sleepwalking, in his nightshirt, which was rather racy stuff for Victorian times, and the crime was always unbelievably complicated. In mystery novels, I mean, nobody ever just grabs the vase and runs, or shoots somebody in a fit of temper, and at the very end, when you think you've got it all figured out, there's one last plot twist, and the crime's always very carefully thought out, with disguises and alibis and railway timetables and they have to include a diagram of the house in the frontispiece, showing everyone's bedroom and the library, which is where the body always is, and all the connecting doors, and even then you don't have a prayer of figuring it out, which is why they have to bring in a world-famous detective."

Rules of Victorian mystery novels as explained by Verity Kindle
in To Say Nothing of the Dog by Connie Willis