SHOULD THE bride's grandfather's live-in girlfriend be sent a corsage? Is the matron of honor allowed to be pregnant? Must the bridegroom's mother and step-mother sit in the same pew, and if so, does the father set between them? Suppose the bride has 12 friends who want to be bridesmaids, and the bridegroom doesn't have enough male friends to supply as many ushers.

Does the bridegroom's son sit at the bridal table, or at the parents' table? After the bride has danced with the bridegroom, her father and her father-in-law, can she dance with her mother's husband, who helped rear her before she dances with the best man?

This is a sampling of the kind of questions Miss Manners gets every spring, just when she could otherwise be enjoying the first warm weather. Being a good sport, she usually anwswers them. (Some sample answers are: Yes, no, why not, it depends on if they speak, and, oh, go ahead.)

But there is a l imit, and Miss Manners thinks she may have reached it. Such silliness has got to stop.

The supposition behind these questions is that a wedding is a set piece, with rigidly prescribed roles, that the wedding party must be ruthlessly cast to fit the parts, and, as in the way of the theater, too bad for those who won't do.

What is the historical precedent for this series of tableaux? Miss Manners, being a scholar, is aware that wedding customs are a jumble of evolving traditions, and that even the proper Victorian wedding was much more a part of the bride's family's own style of entertainment than an abstract law of correctness for all.

In fact, the only wedding custom with a pretense to long tradition and universality, that of public checking up on the consummation of the marriage, seems to have been dropped. Miss Manners can't think why.

The pattern that so many modern brides apparently have in mind can be traced to Hollywood, Calif., circa 1948. According to this, the bridal couple was not allowed to have step-parents, children, more than one grandmother apiece, or more than one grandfather among them. The small size of the family cast was compensated by the number of places available in the wedding party to friends, provided these friends were young, unmarried and of uniform height.

This is a manageable group, and any director can arrange it into decorative patterns. In the processional, the bridegroom and best man come in at the side, and up the aisle come, in order, the ushers, bridesmaids, ring bearer, flower girl and bride with father; or in a Jewish wedding, bridegroom flanked by his parents after the ushers, and bride, flanked by her parents, after the bridesmaids or flower girl.

In the recessional, the bride goes on the arm of the bridegroom, followed by female attendants paired off with male attendants.

In the receiving line at the reception, the hostess, who is the bride's mother, greets guests first, then the bridegroom's father, the bridegroom's mother, the bride's father, the bridal couple and the bridesmaids. The fathers may agree to go get themselves a drink instead.

At a seated wedding breakfast, the bridal pair sit together with their attendants at one table, while the parents (again with bride's mother as hostess and bridegroom's father as ranking male guest) sit at another table with the clergyman and his or her spouse, the grandparents and other close relatives.

So what happens if you have more people than you need for some roles, such as mothers, and a few for others?

Dear brides, you rewrite the script to fit the company. You group your relatives as makes sense to you and then, in terms of their closeness to you and toleration for one another, and you arrange a wedding party that includes your friends, whatever their size, shape and number.

If you complain that this is not correct or traditional, Miss Manners will come around and check up on you the next morning. MISS MANNERS RESPONDS

Q. To me, the cut-and-shift American way of using a fork is silly and I find the tines-down English way to be inefficient.

I hold my fork mainly in my left hand where there is food to be cut up or pushed onto the fork with my knife. The tines are up, and the handle rests on my ring finger and first knuckle, with two fingers and the thumb on top. I hold the fork the same way in my right hand for certain foods.

On several occasions , remarks have been made about this alleged peculiarly of mine. I respond by making it clear I know the alternative. You may say that people who make such remarks have more to learn about manners than what they assume I do not know. Would you, however, advise me to change a comfortable habit just to earn the good opinion not only of those who question me about it, but also of people with the decency to be quiet?

A. You, sir, are an anarchist, and Miss Manners is frightened to have anything to do with you.

It is true that questioning the table manners of others is rude. But to overthrow the accepted conventions of society, on the filmsy grounds that you have found them silly, inefficient and discomforting, is a dangerous step toward destroying civilization.

Q. I'm a secretary -- not the cute, young kind, but a mature woman, married 44 years, who spent 25 years as a housewife. I was always careful to avoid qualifying my title as in "just a housewife." I'm equally careful to avoid saying "just a secretary."

However, it seems that either title demeans one socially. The other night, a young woman asked where I worked and was very impressed when I told her the name of the prestigious institution. She immediately wanted to know what I did there and when I told her that I was a secretary, you would have thought I had suddenly come down with a terminal disease. Even worse were her parting words, "Well, even though you're just a secretary, it must be an interesting place to work."

Miss Manners, I'm so tired of wearing this scarlet S. Can you suggest any way to avoid this situation?

A. "Just a" is, indeed, an offensive qualifier, and the only sure way to avoid hearing it is to stay away from rude people. That is hardly practical in this world, however.

You know that Miss Manners does not alllow her readers to answer rudeness with rudeness. But you might say, with polite pride, "Oh, no, you're mistaken, I started out as just-a-secretary, but now I'm a full secretary."