DEAR MISS MANNERS: When does a gentleman offer his arm to a lady as they are walking down the street together?

GENTLE READER: Strictly speaking, only when he can be practical assistance to her. That is, when the way is steep, dark, crowded, or puddle-y. However, it is rather a cozy juxtaposition, less compromising than walking hand in hand, and rather enjoyable for people who are fond of each other, so Miss Manners allows some leeway in interpreting what is of practical assistance. One wouldn’t want a lady to feel unloved walking down the street, any more than one would want her to fall of the curb.

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1979-04-19)
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Collected in Miss Manners’ Guide to Excruciatingly Correct Behavior, Part 3 "Basic Civilization," "Common Courtesy for All Ages" (1983).