The desire to reduce any body of knowledge to something that fits on a T-shirt -- or, in the case of academic knowledge that's subject to testing, on the cuff of a real shirt -- is understandable. So Miss Manners tries to be patient with the nice people who tell her that her noble field is, "after all, just a matter of common sense."

So much for the world's fascinating variety of cultural and ceremonial traditions. So much for the ever-developing interplay of behavior with changing social thought and technology. So much for the ethical and practical task of applying conflicting obligations to the complications of real situations.

Well, common sense certainly helps, Miss Manners admits. You just wouldn't want to be stuck with it alone in a strange situation where you don't want to offend people of whose customs you know little or nothing. But even then, it couldn't hurt.

Common sense might have the sense to tell you to admit your ignorance and plead for etiquette instruction instead of bludgeoning your way through, unaware of the horrifying signals you may be sending.

So where is all that common sense in common situations, where it is so badly needed? Miss Manners is exasperated to find it in remarkably short supply when it comes to the simplest and most obvious matters.

Example No. 1: The failure to answer invitations, especially formal ones, usually issued for weddings, is as widespread as it is insulting to the hosts, for whom it has serious consequences.

Therefore, Miss Manners used to think that only the callous would omit doing this.

There are plenty of such folks around, who brazenly tell those kind enough to invite them that they have no way of knowing if they will feel like attending on that date. But there are others who seem genuinely puzzled about what to do. If a reply is not specifically requested, or there is no reply card enclosed, or it comes with a return envelope but no stamp, they figure that no reply is warranted. One Gentle Reader said that since she was not planning to go and the reply card asked for the number of people attending and a meal choice, she had not used it or otherwise answered the invitation.

Example No. 2: People who receive announcements of graduations, engagements, weddings or other happy events, when there is no invitation involved, ask Miss Manners whether they can ignore them or are obliged to give a present, as if these are the only possible choices.

Example No. 3: Having received presents, sometimes in the form of checks, the recipients plead that the donors must realize that they are too busy to write letters of thanks, or that they have a year in which to do so, or that it would be offensive to do so at all, as this would imply that the generosity was calculated for an expected return in the way of gratitude.

And these are people who seem to mean well and are actively inquiring about what would be proper. Where, Miss Manners wonders, is their common sense?

Shouldn't it tell them that all social overtures require and deserve an answer? If a friend said, "Do you want to go to the movies tonight?" or a colleague suggested getting a cup of coffee, they might accept or decline -- but would they remain silent?

Do they also greet friends' happy news with silence? Don't they respond with congratulations?

If they send a present, are they ashamed of themselves as selfish if they want to know right away that it arrived and was appreciated?

Etiquette can supply the form and the frills for such responses. But it does need to build on some common sense.

Dear Miss Manners:

Is it bad manners to be in a restaurant and, after a meal course, pile up your dishes to make it a bit easier for the waitress? Would they be insulted by this? My wife always does it, and I would think they would appreciate the help and thoughts.

Would you be insulted if one of your clients or customers pitched in and started doing your job for you?

Before you say, "Great, then I wouldn't have to do it," Miss Manners asks you to consider the possible results. Suppose that person bungled the job? Suppose the boss caught you foisting off your work?

You and your wife mean well, but piling plates is improper table service. And you wouldn't want the dining-room manager to think that you were reduced to piling up your plates because the waitress had neglected to clear them in good time.

Feeling incorrect? E-mail your etiquette questions to Miss Manners (who is distraught that she cannot reply personally) at MissManners@unitedmedia.com or mail to United Media, 200 Madison Ave., New York, N.Y. 10016.

(c) 2004, Judith Martin