IT IS becoming the custom now for people who are already grown up to get married.

Wedding etiquette has always been based on an assumption of youth and innocence on the part of the chief participants, and this is what gives traditional weddings their peculiarly bittersweet appeal. Grownups were not expected to get married, or at least to have any fun doing so. Miss Manners doesn't see why not. Weddings between people who are well acquainted not only with life but with each other should be fascinating occasions.

They should be equally festive, but different. Grown-ups should not make themselves silly by following procedures designed for children. There is nothing wrong with an adult birthday party, for example, but the participants do not wear Mary Janes or short pants, and do not play Pin the Tail on the Donkey (Spin the Bottle is another matter) or pop balloons.

It is still possible to haver a good time. One's last marriage may be far greater cause for celebration than one's first.

Let us consider some of the differences between grown-up and child marriage couples.

Grown-ups usually have: children, toasters, grown-up friends, flatware, formed tastes. Grown-ups usually do not have: friends who will dye shoes to match or wear colored dinner-suits, the sense of permanence required to have towels monogrammed, parents who say, "Oh, what the hell, go ahead. We want this to be something you'll remember all your life."

Grown-up women do not look cute in white tulle or organdy candy box wrappings. Miss Manners has never felt that white wedding regalia should be issued only to certified virgins, although one still needs to consider how big a laugh one will get by suggesting symbolically that the package has never been opened. You know your wedding guests better than Miss Manners does. But for grown-up brides, regardless of previous marital or other escapades, hats, flowers or hair are more becoming than little princess wispies.

It is stretching the imagination far enough to suggest that the father of a young girl has had any hand in disposing of her hand, but the idea of a father's giving away a grown woman is ridiculous. The close relatives whose cooperation adult bridal couples most need are their children. If anyone is to be given a role in the ceremony, it should be they.

The pretense that a wedding means that a fledgling is leaving the nest is also even less plausible than for youth marriages, and therefore Miss Manners believes that it is inappropriate for grown-up brides to have their parents issue their invitations for them. As that is the form for the traditional formal invitation, Miss Manners prefers skipping that approach altogether -- rather than altering the wording to make it sound as if the couple is giving itself away -- and writing real letters: "Dear Rosemary,

Philpott and I are being married on Saturday, June twentieth, at four o'clock at the Town and Suburb Club, and we would be so pleased if you would be with us...."

If the couple simply cannot stay away from the engraver's, they should issue invitations to the wedding reception as they would to another formal party.

The effort of writing out the invitations is compensated by the reduced necessity of writing letters after the wedding to thank someone for the lovely electric wall ornament. No one has to fork over more than one major present, so it is not necessary to give them for multiple marriages. Grownups getting married not only don't need electric hot plates and linen towels, but are likely to be in dispute over which of them should throw awary his or her sets. Only intimates, who know what might be useful and pleasing, and who take pleasure in the idea of making these people presents, need bother.

Of course, the chief advantage of being a grown-up is that one can do what one wishes, including disregarding all sensible advice.

Q: I believe your opinion of breastfeeding in public was a bit simplistic in that it failed to distinguish between nursing per se and exhibitionism, which a minority of nursing mothers practice while they feed their babies. It is hard to understand why you would oppose discreet public breast-feeding (it can easily be done so discreetly that the only obvious sign of it is the swallowing noise) any more than you would oppose bottle-feeding in public. I feel etiquette is rather poorly served by social pressure to either bottle-feed (known to be nutritionally and medically inferior to breast-feeding) or remove oneself from society.

A: Miss Manners knew, when she took up the subject of public breast-feeding, that she was going to end up accused of depriving hungry infants of warmth, love and sustenance. This was not her intention.

Nor does saying a thing is innappropriate in public constitute "social pressure" against its being performed at all; if so, the human race would have ended some time ago.

Exposing the female breast, for any purpose other than getting a suntan on southern French beaches, is considered an exhibition, which is not to say that it should not be done when that is the intent.

Yor are asking about mothers whose intent is only to feed their babies. In that case, Miss Manners' only objection about doing it discreetly is the fear that babies don't breathe well under ladies' sweaters.

Q: How should one conduct oneself in the presence of another whom one has known for an extended period in another place, yet never spoken to? Let me explain.

While boarding a bus on my way home from work recently, I noticed on the same bus a fellow with whom I graduated from law school last June. Although we attended many of the same classes for three years, we never spoke to one another (being both true gentlemen, it could not have been otherwise, since we were never properly introduced). Now that it appears that we will be riding the same bus home, it seems a bit silly to continue ignoring one another, particularly should we ever end up sharing the same seat. Could you arrange an introduction?

A: Allow Miss manners to introduce you to a principle:

"The roof constitutes an introduction." This does not apply to every roof. It does not apply to a bus, for example, although it does apply to a ship. It does, however, apply to an academic institution. Theefore, it is as proper for you to say, "I believe we were in class together" as it would be, if you had been "properly introduced," to say, "I believe we met at the Winter-greens' last Christmas."