They were standing under a tree each with an arm round the other's neck, and Alice knew which was which in a moment, because one of them had "DUM' embroidered on his collar, and the other DEE.' "I suppose they've each got "TWEEDLE' round at the back of the collar," she said to herself.

(I've updated this slightly, since nobody embroiders collars anymore. For an even MORE updated version, click here.)