Marriage is an alliance entered into by a man who can't sleep with the window shut, and a woman who can't sleep with the window open.

Shaw’s epigram treats marriage less as romantic fusion than as a practical pact between incompatible temperaments. The window image turns intimacy into a domestic problem: two people share a room, yet their basic comforts and habits pull in opposite directions. The humor depends on recognition—marriage often forces continual negotiation over small, bodily, everyday preferences that symbolize deeper differences in outlook and control. By calling it an “alliance,” the line also hints at marriage as a social contract: cooperation is necessary, but harmony is not guaranteed. The joke’s sting is that companionship may be built on managing friction rather than eliminating it.