A wedding invitation is beautiful and formal notification of the desire to share a solenm and joyous occasion, sent by people who have been saying, "Do we have to ask them?" to people whose first response is, "How much do you think we have to spend on them?"

The making of a wedding guest list is therefore a momentous ritual. It is also usually the occasion of the first breakdown of pleasantness between two groups of people who have been trying to pretend that it is perfectly natural, and even wonderful, to find that you are about to become related to a bunch of strangers selected through inexplicable whim.

The first matter to be fought out is the size of the wedding. Weddings come in three sizes: private, small and large. No one ever says, "We thought we'd like to have a medium-sized wedding."

A private wedding is designed to exclude certain near relatives who can be told, "We didn't want any fuss--just our parents," in the hope that they will not find out that 35 people attended, some of them not so closely related as they.

A small wedding is one everyone describes during the planning stage as "75 at the absolute maximum, I don't care what the cut-off is, we just can't have any more." It is attended by 150 guests.

A large wedding is described as "Well, so what? We're really going to do it up right, and have all the people who mean something in our lives." The list grows from 200 to 500 because of such discussions as "What about the Whittlesmiths--those people you met on the cruise that time? They're good for a pair of silver candlesticks, at least."

Whatever the number is, it is then divided evenly between the two families, with the bride's family secretly reserving the right to throw in a few extras, a privilege strictly denied, in both meanings of the word, to the bridegroom's family. The bride also persuades the bridegroom to put their mutual friends on his list.

Each side then figures out how to fill its quota. Miss Manners' guidelines for the task are:

Cut-offs by category (no second cousins, nobody under 12) are easier than by individuals. They do not preclude grumbling, but it is more general ("If she hates children so much, why is she getting married?") and therefore less offensive than the specific ("Of course, he's always hated Daniela since that time she told on him in the playground").

Anybody who writes "and family" or "and guest" on a wedding invitation deserves whatever she gets.

A bride or bridegroom's antipathy towards her or his relatives or parents' close friends carries no weight whatsoever.

Statements such as "Let's go ahead and ask them--they'd be hurt if we didn't, and they live 3,000 miles away and aren't well and are having financial troubles, so there isn't a chance in the world they'll come" never turn out to be true. These people will mortgage their cat, charter an airplane, and get there.

One should never yield to the plea to be allowed to bring a date. A wedding reception is not a nightclub, to which one brings one's own guests. If a couple cannot spend one afternoon or evening apart, why don't they get married themselves, because--

One cannot invite one member of an engaged or married couple without the other, no matter how many grudges one has against that person. (For the purposes of social harmony, Miss Manners usually assumes that people who share a household are secretly engaged or married.)

Following these rules, one should be able to make up the guest list without having to call off the occasion to which they are invited. It is, after all, the sharing of confidences between the bridal couple about which of their own relatives are likely to make fools of themselves at the wedding that is the true first step toward creating a new family. MISS MANNERS RESPONDS

Q. Ten years ago, my mother passed away. Two years later, my dad remarried. I call her by her first name. My children call her "Grandma."

When introducing her to my friend, what do I say? "This is my Dad's wife"? "My stepmother"? No way will I say "This is my mother" or call her "Mother."

Yet I am very fond of her, and respect her as well. She and my dad feel it is very disrespectful to say she is my stepmother. Please answer as soon as possible, as this is causing some friction in the family.

A. My, my, my. Will you all please stop putting your own emotional connotations onto perfectly good words? What is the good of devising conventional terms for everyday use if people insist on rejecting them for idiosyncratic reasons?

It is nonsense to say that "stepmother" is a disrespectful term. It describes a pefectly respectable relationship, and Miss Manners refuses to concede to it that fairy-tale association with wickedness. It seems to her that assuming the task of motherhood to someone with whom one has not been associated through birth is an honorable undertaking.

Neither does she think that using the shorter term of "mother" to describe a stepmother or a mother-in-law takes anything away from one's original mother.

Among these prejudices, Miss Manners advises deferring to the preferences of one's elders. You are a grown-up woman, and it seems to Miss Manners that you can manage to choke out describing her as your mother, if you are unable to convince her that the affection and respect in which you hold her has made the term "stepmother" very dear to you.

Q. Please tell me how to deal with a person who uses his or her excessive politeness to make me appear rude or greedy.

I know, in this day and age, and what with the way people behave, it is hard to imagine someone being too polite. However, one person I must deal with frequently insists I take the bigger piece of dessert, that I go first and that I should have my way.

Frankly, Miss Manners, at times it is nice, but when this person does this continuously, particularly when others are present, I feel I am being made to appear as an inconsiderate, greedy slob.

Is there a nice way to tell this person that just once, I would like to see them push me out of the way and grab for the piece of sweet potato pie with the bigger scoop of ice cream on it? It would at least make me feel this person is human and that would make me feel more comfortable when around him or her.

A. Did Miss Manners hear you correctly? Are you seriously trying to tell her that the opposite of "polite" is "human"? Should she conclude that the opposite of "rude" must be "inhuman"?

No, no you don't mean that at all. What you mean is that you have found yourself engaged in a contest of politeness, which you are losing. Surely you have detected a note of sarcastic exaggeration in the other person's behavior, which makes you suspect it is not being done in your interest.

Such behavior is, Miss Manners regrets to tell you, strictly within the laws of etiquette. As Miss Manners keeps pointing out, manners have other uses than the popular one of "making others feel comfortable," and challenging someone to such a contest, for whatever motives, is one of them.

To win, you must exaggerate your politeness and keep saying, "No, no, I wouldn't think of it--I couldn't possibly enjoy this pie if I thought you were having less," etc.

Please note with what sympathetic cooperation Miss Manners has treated your rather odd request. If she is being too polite and you are really only asking that the standard of good manners be lowered so that you can relax into rudeness yourself, you are on your honor to feel ashamed of yourself.

Q. I am the invitation chairwoman for a high school class reunion party. Should the invitations be addressed only to the class members, or to members and spouses? The invitations do say that guests are welcome at the party.

A. Unless this is a reunion of the February Class of '83, there is no way that you can predict with accuracy who has the same spouse as he or she did at the last compilation of names. Indeed, a great deal of the fun of having a reunion is to discover what has happened to everyone's living arrangements since the last time you checked.

As this is a reunion of the class, it is no slight to family members to address only class members, allowing them to choose whom they will bring. If you ask them to submit the names of their guests, or simply learn by introducing yourself at the door as people arrive and present their friends or whatever, you will be the most popular person at the reunion as the evening wears on.