THE SOLEMN rule that one must never break an engagement just because something better has come along does not, of course, apply to engagement to enter into holy matrimony. A person who would break an engagement to dine for reasons other than sudden death is a cad, but for an engagement to be married, the excuse "I don't know, I just don't feel like it" will do.

Miss Manners does not mean to suggest that breaking such engagements is not a serious, delicate matter. When doing so, one cannot be too careful of the tender feelings of those most concerned - the caterer, the bridesmaids and the mother of the bride.

Weddings are, after all, public occasions, and not merely opportunities for lovers to show off. Society goes so far as to declare the caprice of falling in love to be "healthy" when it can lead to conventional marriage, and "a sign of something" when it does not. Nobody ever packed two single 25-year-olds of opposite sex but equal education and family background off to a psychiatrist to find out the hidden reasons for their falling in love.

Therefore, when one is jilting one's publicly affianced, it is only fair to do so at the convenience of that part of society that has become involved.

This comes to mind because Miss Manners was told a story about a perfectly dreadful bride. It seems that she cancelled the wedding while all her friends were busy scraping up the money to pay for the bridesmaid dresses they had chosen and then, when everybody thought it had all died down, upped and married the man without their attendance.

That last act wiped out any possibility of sympathy for a panicked young girl who suddenly sees herself ruining her life, and bolts. What remains is someone whose caprice struck her closest friends with bills for dresses they didn't want.

Let us consider how such a case should be properly handled. One evening, three weeks before she is to be married, Daffodil tears her eyes away from her 40-page check list of Things the Bride Must Do and takes a hard look at Ethelred, her betrothed. She never noticed before how his dinner tends to remain on the tip of his moustache, and besides, his mother turned in a guest list with "and family" added to all the names. Life with him, she realizes, would be as flawed, or as cloudy, as the diamond he gave her.

As a considerate member of society, she immediately finds out how many of the wedding commitments are irrevocable. If the disadvantages are outweighed by the refunds, she cancels it. If she then notices that Ethelred does not look as heartbroken as she had imagined, and decides to uncancel him, she re-schedules the wedding with the original participants at a date convenient to them.

Suppose, however, that the revelation occurred two nights before the wedding. What does the considerate bride do? Miss Manners would never advise a woman to marry someone she doesn't like just because the arrangements have been made. She could marry someone else, instead.

Or she could jilt him at the altar, which would mean that the party would go on, with a little more spice, even, than if the wedding had taken place.

MISS MANNERS RESPONDS

Q. Is it right to tell a woman that her slip is showing, or she has a run in her stocking? I figure she might want to know, but I don't want to be offensive. If I had a spot on my jacket, I would want her to tell me before I went in to see the boss, for instance, so I could fix it

A: Yes, But suppose you had a spot on your tie, which everyone knows is impossible to remove. First you would try to fix it, succeeding in making it worse, and then you would go in knowing that you were a Man With a Spot on His Tie. Consider, before you point out a problem, whether it is solvable (or, in the case of your spot, soluble) before mentioning it. The slip can probably be hiked up. The run cannot be undone.

Q: I was at a dinner party the other night, where a lot of silver was spread out at each place, for many courses. I know you taught us which fork to use when, but I forgot. Kill me. My question concerns the next step after one uses the wrong fork. I realized, by looking at the hostess's place, that I would never come out even, and , in fact, I had to ask for something to eat the salad with. If I promise to go back and study fork order, would you consent to tell me what to do about mistakes?

A: Miss Manners is a tolerant soul, but feels that you are concealing from her the extend of your mistake. If, for example, you ate the fish with the salad fork, why did you not have the fish fork left, with which to eat the salad? Miss Manners is always ready to receive back a sinner, but not those who steal the forks. However, she will let you in on the secret of correcting mistakes, if you promise not to tell anyone she said so. Lick clean the wrong fork you have just used, and slip it back on the tablecolth while no one is looking.

Q: In a recent novel about Washington, an important woman is giving a dinner party at which she has several round tables, which are, as one character notices, of varying degrees of importance. This does not suprise me. But I was amazed at this woman's telling her guests that she was terribly sorry, but she had to "break up" husbands and wives in the seating arrangements. Aren't husbands and wives normally separated at Washington dinner parties, the way they are elsewhere?

A: In Washington, as elsewhere, husbands and wives tend to know the correct versions of the stories their spouses are telling inaccurately, and therefore civilized dinner conversation is impossible unless there is a decent distance put between them. But Miss Manners has enough trouble making live people behave properly, without taking on fictional characters.

Q: What does the term "turning the table" mean, and how is it done? Is it, in fact, still a social custom, or is it archaic?

A: Turning the table is one of those marvelously civilized customs invented by our ancestors who knew, in their wisdom, that no one person should be forced to endure the conversation of the same individual for an entire dinner party. Conscientious hostesses still practice it, although they must pretend they are not, because now we are all supposed to believe one is fascinating proving you get to know him well enough.

Whereas in the old days, a hostess could prefectly well confide to Mr. Crawbeard on her right that she was now planning to talk to Mr. Fiddlediddy on her left, she must now be more subtle.

Crawbeard has been explaining to her, from the soup course until halfway through the roast mutton, the troubles he has been having with his automobile. She should then turn to Mr. Fiddlediddy, who hasn't heard a word of this because he has been busy making plans for slipping away after the dinner with Mrs. Boltenbrook on his left, and say in a loud voice, "Don't you agree, Simon?"

If he has any sense of propriety, he will agree, after which he and his hostess may begin a conversation on any subject they choose, while Mr. Crawbeard asks the lady on his right how many gallons she gets per mile, and Mrs. Boltenbrook begins sizing up the gentleman on her left. CAPTION: Illustration, no caption, by Charles Dana Gibson.