"'s plain clothes, see?" said Nobby.
Vimes straddled the thin line between hilarity and horror. You might think it's only so thin because "hilarity" and "horror" both start with H and sound nice together, and otherwise no one would think they were even on the same continent; but they really do touch sometimes, the way the hand of someone in a car waving connects with the jaw of the motorcyclist being waved at. The line is especially thin between the legs of the incurably knurd, so's they can straddle it more easily.
"Did I make the word 'plain' plain enough?"