Still having trouble telling the Petersburger from the Moskvich? Try this simple experiment.
Take one Petersburger and one Moskvich and place together in an empty room. Lock the door. Bar all windows. Sit down outside and wait.
When half an hour later a head squeezes its way out of the ventilation shaft, it can belong to only one of the two inmates: the Moskvich. He brushes off his hair, smooths down his suit. "Sorry," he says in explanation; "Got an appointment to keep."
Now unlock the door. Go away. Come back half an hour later. Look inside: the Petersburger has not moved from where he was last seen - seated comfortably inside a dense cloud of cigarette smoke. Asked why he hasn't taken the opportunity to escape, he replies: "Escape! Escape! Can't you see I'm expecting guests. Someone could drop in and see me any moment now."
Lock the door again. Come back two hours later.
There are now two Petersburgers in the room. Two Petersburgers drinking tea, amidst two separate clouds of smoke. The original specimen has been supplemented by a friend - who has dropped in off Nevsky Prospekt through the ventilation shaft. In the tradition of the impromptu St Petersburg guest, he has not come empty-handed.
Go back out. Return three days later. There are now five distinct clouds of smoke in the locked room...
Now check on the Moskvich's activities and whereabouts. In the last three days he has: sold his flat (twice), bought three new cars, married, divorced, formed a new political party, married and divorced again, and written a postmodernist account of the half an hour he spent in your locked room.
Now do you see?