Q. What does one do when the bearded gentleman you may be talking to at a cocktail party has a few crumbs of cake or quiche in his beard? Likewise, when a person you know, perhaps but slight, has spinich stuck between his or her front teeth? Do we gently tell them, ignor it all, or, in the case of the beard, lean over and brush it off?

A. You are talking about a problem of such complex delicacy that it requires exquisite tact to judge each individual instance. Miss Manners can see that you realize that there can be no solution applicable to all such situations. You do not, for example, suggest leaning over and picking the spinach out of someone's front teeth.

Here are some guidelines for making a judgment:

1. Is this something that others are likely to notice?

2. Is this something that the victim is likely to realize after it's too late to do anything about it?

3. Is there a way of correcting the problem without seeming to take it seriously?

For example, one of the severest cases of That Sinking Feeling is brought on when one returns home, satisfied that one has been unusually witty and merry at a dinner party, and then sees, in the bathroom mirror, that one has spinach on the teeth. It does not take long to calculate when the spinach was consumed and for how long afterward one displayed one's triumphant smile.

This is not a nice feeling.

In the case of a crumby beard, one may possibly assume, especially if one has a quiche-colored beard, that no one else has noticed it.

Miss Manners would therefore point out the spinach but ignore the crumb, unless the crumb could not possibly have gone unnoticed -- if it were an inch by an inch, say, and covered with tomato sauce.

Now about how to do this: The correcter should seem to have some doubt, in order to convince the sufferer that the problem is of minimum noticeability. "Excuse me -- I can't quite tell. Is there something on your tooth?"

Or perhaps the brushing off of a crumb could be accomplished with a slightly flirtatious gesture. (You begin to see why delicate individual judgment is required.)

And if you think your examples are difficult, what about the lady of Miss Manners' acquaintance who only realized after a tea at the State Department -- in the days when pants were considered dressy -- that her fly had been open all the time and nobody had told her? She still holds this against each and every woman at that tea.

Q. When you address an envelope, it is correct to say Mr. and Mrs. John Murphy, using the man's first name or initial. When you write your note on the inside, you usually say Dear Mary and John -- or should you say Dear John and Mary? We've had a discussion about whether it is correct to use the man's name first, or the woman's.

A. The woman's. Miss Manners refuses to get into the discussion of why.

Because the minute explanations go beyond the conventional into an exploration of possible reasons and symbolism, there is going to be big trouble.