“Well, it’s just a question of where people happen to keep their keys. You go up to your bedroom, and perhaps you like to lock your door in case anybody comes wandering in when you’ve only got one sock and a pair of braces on. Well, that’s natural enough. And if you look round the bedrooms of almost any house, you’ll find the keys all ready, so that you can lock yourself in at a moment’s notice. But downstairs people don’t lock themselves in. It’s really never done at all. Bill, for instance, has never locked himself into the dining-room in order to be alone with the sherry. On the other hand, all women, and particularly servants, have a horror of burglars. And if a burglar gets in by the window, they like to limit his activities to that particular room. So they keep the keys on the out side of the doors, and lock the doors when they go to bed.” He knocked the ashes out of his pipe, and added, “At least, my mother always used to.”

“You mean,” said Bill excitedly, “that the key was on the outside of the door when Mark went into the room?”

“Well, I was just wondering.”