The New York Times report of a well-known British conductor's assisted suicide alongside his wife has a passage that illustrates a rare and tricky challenge.

Sometimes, an error in a public sign surprises me because of the context.

Even the notoriously smarty pants "Countdown With Keith Olbermann" (and I say that with affection and admiration) makes a mistake that bedevils many of us. In a caption promoting Friday night's story about Sarah Palin's interview, the "Countdown" crew on MSNBC mixed up whose (possessive) and who's (contraction for "who is").

We have only one rule to remember: Possessive pronouns never use an apostrophe.

Two advertisements caught our eyes this week. They have apostrophe problems, but different ones. The first, pictured below, falls into the old trap of using an apostrophe to make a plural. The shopping center mentioned in this ad, which came as a direct mail flier, is Clayton Corners (multiple corners).

The second ad appeared on television. It has a more unusual apostrophe problem, as you can see below. We use apostrophes to make nouns possessive, a vestige of a time when English was a more inflected language, when words changed form to indicate their function in a sentence. Pronouns still change form when they are used in the possessive case, but nouns merely take 's or ' . I wonder if the ad writers feared the apostrophe. Or maybe the apostrophe was just omitted inadvertently.

At the risk of sounding like a schoolmarm, I am going to repeat something that we learned early in elementary school.

Here are the simplified rules for making a common noun possessive.

• For singular nouns, add 's. man's friend, girl's dress.
• For plural nouns ending in s, add ' (apostrophe only). boys' game, dogs' leashes.
• For plural nouns not ending in s, add 's. men's friend, children's games.

There are, of course, variations on the rules, but these rules will carry us through most cases.

Update: A commenter questioned whether the shopping center mentioned in the first ad was indeed Clayton Corners. It is. Below is a photo of the shopping center's sign.