IT IS WITH inexplicable pride that people describe themselves, their homes and their dinner parties as "casual" or "informal." From the tone of voice in which just about everyone says, "Oh, of course, it's to be terribly informal," or "We're always casual," you would think that Amereica in 1978 is like the court of Louis XIV, and that they, alone, have introduced some spontaneity.

Properly, "informal" means "black tie," as opposed to "formal," which is "white tie," a rather charming style now kept alive only by magicians and pianists. Instead of either of those, we now have a dreadful thing called "black tie optional" for occasions before which one was expected to wash one's ears extra carefully, and for all other occasions, a worse style that Miss Manners calls "optional." That is what others are calling "casual" and "informal," by which they mean that they don't plan to take any troubel.

When Miss Manners longs for style in daily living, she is by no means thinking only of styles that were associated with the rich. Until recently, both rich and poor distinguished between their work clothes and "Sunday best," between meals on the run and company dinner, between the back room and the front parlor. This distinction was not an affectation, but a special effort made out respect for an occasion, or for esthetics difficult to maintain daily.

Now we have a member of Congress complaining that he is expected to go to the trouble of wearing a necktie to the U.S. House of Representatives. Miss Manners understands that a member of this institution may know better than she whether that is a place to be treated with respect, but would expect him at least to gloss this over in public, as do his colleagues.

The excuse people use for being sloppy is that this is more comfortable. Actually, people often feel better when they are uncomfortable, which is why high heels were invented. Miss Manners was once asked whether dress at her own dinner party was to be "formal or comfortable," and, given such a choice, replied, "Informal, but uncomfortable."

Also, informality does not necessarily produce comfort. Miss Manners is infinitely more comfortable eating her dinner at a properly set table than jauntily perched on someone's rug.

When it is truly a matter of acute discomfort, Miss Manners is not opposed to the entire society's agreeing to make certain changes in conventional dress or furnishings. For example, Miss Manners always thought the stocking or panty-stocking a fetching garment for springtime, autumn or heated ballrooms, but a menace in really cold or hot weather, and she is delighted at the spread of the "city sandal" which may be worn barefoot in summer, and the leather boot, which may be worn with heavy knee-socks in winter. She would even be willing to listen to arguments in favor of replacing the necktie - if another formal daytime standard were university adopted.

What she opposes is defying prevailing standards. Once the society has agreed what is proper - as in the cases of the boots and sandals, but not yet the neckties - Miss Manners expects people to observe this in public places. Even if it's only the House of Representatives. Miss Manners Responds

Q: Recently I arranged a blind date between two very dear friends. Believe me, I had only the highest intentions for their mutual happiness. However, believing, as you do, in the delicacy of one's sexual feelings, cravings and demands, I had no idea that such a match was unlikely. One of the parties, though, is now revealed to be homosexual. Hew should I advise the other friend? Will it be necessary to cease matchmaking in the New Society?

A: Why would you want to give up just when it's getting so interesting? Perhaps by "the New Society" you are referring to heightened standards in consumerism. If so, rest assured that a matchmaker is not supposed to have pretested the product, and therefore has no responsibility for its performance.

Q: What excuse should one use when visiting a house in the neighborhood - open for sale - when one is not interested in buying but only interested in satisfying his curiosity as to price and furnishings?

A: "Thank you, but I was hoping to find something with less closet space."

Q: My avocation, as well as my luck, requires that I write an occasional business letter. What is the proper complimentary closing? Many are to people whom I do not necessarily respect; that lets out "respectfully." "Yours" is obviously out; I am most certainly not. While I hope I am almost always sincere, "sincerely" seems out of place in a business letter. So does "love" whether agape or filial. How about just "very" or how about nothing?

A: How about "Yr. most humble and obedient servant"? How about not being so literal? Miss Manners signs herself "very truly yours" to acquaintances - believing that "sincerely" alone is as close to nothing as "very" alone - and has not yet been required to surrender herself to any of them.

Feeling incorrect? Address your etiquette questions (in black or blue-black ink on white letter paper) to MissManners, Style Section, The Washington Post.