TWO TRAGEDY of life strikes Miss Manners as so hopeless as to be without redeeming value as an occasion for teaching etiquette. The need to deal politely with a person whose memory and perhaps emotional stability have eroded with age is, in that light, a rigorous course in the exercise of patience and compassion.

Without suggesting that it is a priceless opportunity no one should miss, Miss Manners will go so far as to say that it would be a mistake for a parent who has such a problem, perhaps with his or her own parent, to keep it hidden from the next generation down. You never know when you might be personally terribly grateful to have taught your child to be kind to the senile.

Comparisons with childhood, as when dotage is referred to as a second childhood, are, in Miss Manners' opinion, harmful. That idea suggests more than the similarity of care-taking services which may be necessary; and treating a grown-up, however incapacitated, with parental authority is undignified. It is difficult to remember to observe the forms of independence with someone who has actually become a dependent, but tact demands it. One says, "Oh, do have some more, please," rather than, "Eat up, and stop wasting my time"; "Oops, let me clean that up," rather than, "Now look what you've done."

The place where a grown-up lives is always treated as if it is under his command. One knocks to enter, even if it is not practical to stand upon the reply; and one asks permission to help out, even if that encompasses taking charge of things.

The childhood simile is also harmful in creating expectations that things will improve. Impatience is sometimes justified toward the young, because one expects them to make progress and wants to prompt them along to accelerate the pace. When the movement is instead toward deterioration, an attitude geared toward learning can only be frustrating for everyone. The cover-up remark ("Those appliances are tricky") is of more use here than the instructive one ("Pay attention while I show you so next time you'll know").

One of the worst consequences of memory loss is the accompanying humiliation. The civilized reaction, when anyone is having any kind of embarrassing difficulty, is to fail to notice the problem, and, at the same time, to offer help while apparently unaware that that is what one is doing. In this case, you seem to supply missing names or other words as a conversational gambit. It is amazing how many people think it more fun to play guessing games, along the lines of, "Now, do you remember who this is?" Such people, like those who push their way through parties saying, "I bet you don't remember me!" deserve what they usually get.

As recent memory sometimes goes before what was stored earlier, the most successful conversation is about the past. It generally is, anyway. From the smallest child who fails to understand why he gets a laugh by opening a story with, "When I was little," we are all fond of reminiscing. Under circumstances when one's past is more rewarding to dwell upon than one's present, it is particularly enjoyable. Repetitiousness is not exclusively a fault of age (Miss Manners has the awful feeling she's already said that), and the knack of listening politely to something one has heard before, even a few minutes ago, is a valuable social skill.

The visitor to the impaired elderly might find himself burdened with an increasing amount of the sociability. This could be the one polite opportunity of a lifetime to hog the conversation, with only an occasional rhetorical question. To someone unable to carry on a conversation, it may nevertheless be pleasant to feel that one is part of an apparent exchange.

Miss Manners does not mean to suggest that it all need be a sacrifice. It is no small thing, nowadays, to have someone willing to listen to you, perhaps even sympathetically.

That is not, however, always the case. Teaching one's children to be tactful, patient and unilaterally sociable with the elderly is simple; explaining why an older person who once cherished them is now indifferent or hostile, is close to impossible.

"Grandmamma is not really herself," is the usual explanation, and as good as any. Why, then, one has to treat her as if she were, can only be answered by a lecture on respect. And you certainly do want to teach your children to respect their older relatives, regardless of merit. Q.Q. Recently, we redecorated our Q. home in lovely fall colors, and hung a few of our own paintings (we are both semi-pros in the art area) and an antique Indian rug over the wall above the davenport.

Dear friends, whom we hate to offend but whose taste leaves much to be desired, did a huge -- 5 feet by 3 1/2 feet -- paint-by-number thing of the Last Supper, spending many hours and loving thoughts on their gift. It is to hang, they said, "in place of that old rug on the wall." The colors are garish, and it is surely not our taste in artwork.

Do you have any ideas on how we can proceed without hurting feelings -- but not hanging it? A.A. Miss Manners likes to think A. that friendship is a more beautiful thing than any work of art, but no mere human relationship is worth a life of contemplating a paint-by-numbers Last Supper.

Fortunately, it is your friends who, however well-intentioned, have overstepped the bounds of friendship. No one makes decisions about decorating another person's house, and certainly such presumption does not create legitimate obligations. Hang it in a back room and explain that you find this is a good place to enjoy it, or simply don't hang it at all and tell them you are looking for "just the right place" to do it justice.

By the way, did they worry about hurting your feelings when they insulted your rug? Q.Q. I use a man's budget or Q. shoulder bag (I use the term "budget" in the original sense of "bag" or "pouch") since otherwise my pants pockets appear to be stuffed with potatoes.

While I understand that you might question the propriety of men using shoulder bags in a refined social activity -- given advance warning, I always transfer necessary documents to a slim wallet and leave the budget at home -- I am concerned about the etiquette of men's budgets when one is dining in a crowded restaurant and seated on a chair, rather than a banquette.

Must one keep the budget in one's lap under the napkin, or can one set it on the floor, after surreptitiously hooking the strap around the right front chair leg, or otherwise securing it against unscrupulous individuals who have been known to frequent crowded restaurants?

When one is shopping, can the shoulder strap be passed over the head, leaving both hands free for steering the shopping cart, or should the strap rest on the shoulder, from which it frequently slips unless the cart is steered one-handed?

I realize the purpose of etiquette is to add the spice of minor difficulties to our existence, so I probably should keep the budget in my lap, or slipping off my shoulder.

The obvious solution, to patronize uncrowded restaurants with banquettes and to hire a servant to do my shopping for me, is unfortunately beyond my means at this time. A.A. With whom, pray, are you A. arguing? Miss Manners hesitates to break into such heated exchange, in which you trounce with sarcasm whoever is telling you that you must keep this object on your lap or shoulder, but begs to point out that it isn't she.

Small leather hand luggage, such as a shoulder bag or briefcase, is perfectly proper for daytime and business use by both ladies and gentlemen. (Ladies use tiny bags for evening, for which there is no male equivalent; consider yourself lucky if some lady does not ask you to put her compact, handkerchief, mad money, perfume and comb in your pocket because these bits of satin or metal are nearly useless.)

They are generally worn over the shoulder, but Miss Manners knows of no regulation forbidding their being worn around the neck or parked on a restaurant floor. If you do, feel free to continue the argument without her. Q.Q. Recently, my wife and I were Q. invited to dinner at an old friend's house. The husband and I have known each other for nearly 30 years but seldom see each other as we live quite a distance apart.

I prepared for the evening by thinking nostalgic thoughts and looking forward to continuing my more recent friendship with his wife.

When we arrived, I was surprised to see another couple there, and we were later joined by still others. I had been so looking forward to spending the evening with just my friends that it took some time to get into the right frame of mind. If I had known others would be there, I would not have been so disappointed.

Is it foolish of me to think that they, or anyone, should say something to the effect that others have been invited to dinner as well? A.A. Although your feelings are A. highly flattering to your friends, hosts are required to assume that you would be more amused with other people present. No one ever claimed that etiquette was logical.

In other words, those who entertain only one person or one couple are expected to say, "It's going to be just us -- we're going to be selfish and keep you all to ourselves." Otherwise, as in your experience, it may be assumed that there will be other guests present. Q.Q. All right, all right. I hear you Q. about the thank-you letters. Now tell me what is the proper stationery for children. Do I let them use my good paper with our address on it? It's expensive, and they always mess up the first time. If I have to buy them their own paper, what is right for children? Personally, I hate the bright-colored papers and cute things with funny pictures that people are always giving me, but don't know if it is worth it to have their names put on paper that isn't going to be cheap, either. A.A. Paper with a child's name or A. initial on it is a charming idea, but only after they have used up all the childish paper not suitable to adults that their parents have in their desks. The good house paper of the family is used for the official correspondence of children if and when they have any applications for jobs or schools.

It may also interest you to know that all children are required to do rough drafts of parent-supervised letters, such as thank-yous and answers to invitations. In their private correspondence -- to pen pals, relatives to whom they can complain about their parents, and notes to be confiscated during school -- they are allowed to spell at their own risk.