When addressing me, a small boy, he used the plural of the second person--not in the stiff way servants did, and not as my mother would do in moments of intense tenderness, when my temperature had gone up or I had lost a tiny train-passenger (as if the singular were too thin to bear the load of her love), but with the polite plainness of one man speaking to another whom he does not know well enough to use "thou."

link: Nabokov's Speak, Memory