HEMLINES AND TIE widths are easy to keep track of, compared to dining fashions. This week's avant-garde sauce quickly becomes last week's leftover gravy, and as soon as you can get a reservation in the newest, hottest restaurant, you have gotten that reservation because it is too late.

The couture shows -- in food, that is -- are in full swing in Manhattan; and before you know it, knock-off copies will be available in your local pubs and dinner houses. So even though you live four hours south of New York, you are going to need to know what to be seen wearing on your necktie (which, if you care, should be silk knit).

First, however, you need to know what not to call it. Nouvelle cuisine, like gourmet cooking, has become a term used by only those who are not doing it. With Marriott opening a nouvelle cuisine restaurant (The View) and carryouts advertising the same, you know it is only a matter of months before it hits the vending machines. As for what to call it just avoid calling it anything at all; names are for the masses, not for fashion leaders.

The season's opener was at The Four Seasons restaurant, a party for five or six hundred people to launch the restaruant's new cookbook. The cookbook, was not exactly crucial; in fact, copies were not even available, except for blowups of a few pages on the walls. Besides, with the champagne generosity and the racks of lamb being dispensed like lollipops at Halloween, the party had to have cost more than the royalties from $10,000 copies.

Rather, it was a fashion show, with buffet tables as runways and hundreds of food writers, wine writers, restauratuers and bon vivants acting as fashion commentators.

Don't miss thees oysters in puff pastry," cautioned Bill Rice, who was also making his public debut as the new editor of the International Review of Food and Wine, the slick and important food magazine recently purchased by American Express. Rice was in a position to know, since he had just had lunch at The Four Seasons, besides having dined there several other times in the past week.

Gael Greene, New York Magazine's longtime food critic and contributing editor, had launched at Le Plaisir, while another New York Magazine food writer and contributing editor, Linda Wolfe lunched at the next table. Greene thus had already had a day's ration of puff pastry, covering the soup at Le Plaisir. Bill Rice had not been to Le Plaisir since the night before; but then, according to its owner, Stephen Spector, a reservation is necessary a week ahead of dinner (at $35 prix fixe) now that the city's chefs and foodwriters are stopping in to see the new menu of this tiny food fashion house.

Although it was The Four Seasons, nobody hesitated to loudly tout other restaurants showing well this season. Le Plaisir, with its quail grown on Spector's New Jersey farm and its vividly hued, precision-cut arrangements of tiny quenelles and two-bite fish fillets and vegetable mousse, is currently considered a piece de resistance among New York restaurants. Then there is Joanna's, a cafe said to be more like Paris' La Coupole than the original -- and with better food. The key word on Joanna's menu is "light," as in "a light steak."

Those who were not taliking of new restaurants were talking of their new books. Julie Dannenbaum confessed breaking away from her Philadelphia cooking school for this party simply because she has a new book coming out in the spring. Victor Hazan, whose cookbook author wife Marcella was not there, was talking of his new book on Italian wine. Alex Bespaloff's revised wine book was no longer news, but he and Hazan were comparing notes on selling off their cellar's French wines that had become to valuable to drink. George Lang's book, "Lang's Compendium of Culinary Nonsense and Trivia," was new, too, but then Lang always has a new book to talk about, or a new restaurant or a new dining complex he has designed. He also had a new companion to introduce, but Jennifer Harvey is new to New Yorkers, but not to Washingtonians, who knew her as before as The Post's Resolute Shopper.

Craig Claiborn didn't seem to be talking much, but that was because people are always talking to him. Also talking little was sculptor Richard Lippold, who had done the Bar Room's focal art, a sculpture of 3,000 brass rods suspended fron the ceiling. He had just arrived from Italy, and was absorbed by the California wines that were interesting than the champagne nearly everbody else was drinking.

When the general flow of conversation was not interrupted by chewing, the talk was of the food that had just been chewed. "Have you tried the pureed white beans?" one might ask.

"Where was the alarmed response of somebody who might be left out of the New York experience of white bean puree. And off he would dash to the bean table. Roast rack of lamb stuffed with leeks and spinich was on the rear balcony buffet, but it nevertheless drew a crowd who found a way to eat rack of lamb and drink champagne standing up, no mean feat. Calf's brains in mustard crumbs with fried capers were in a far corner, but the grapevine eventually sent a steady stream of fans to devour them. That led one right to the saute of chicken, oysters and shrimp with fresh shiitake mushrooms. The sliced rare beef and civet of venison seemed old-hat, so people just ate them without talking about them. But a certain revivalist spirit was evident in the reaction to the tartar steak. People raved about the tartar steak as they had never seen the likes of it before. George Lang called it the best in New York, and thus sent people scurrying to that table while Four Seasons owners Tom Margittai and Paul Kovi honored Lang by bringing him another portion of the raw beef to him.

As usual, Lang had pinpointed a culinary trend: return to tradition. Four Seasons chef Seppi Renggli verified the sighting, noting that chicken pot pie and pot roast are on the upswing at The Four Seasons. But these, like knit ties, are modernized versions, with rare chicken and crisp vegetables, thinner sauces -- and more butter than ever in the pastry.

Nouvelle or vielle, a pot pie by any other name will smell sweet this season.

Here they are -- the freshest food fashions: INS 1. Late lunch, starting no earlier than 1:30 2. Homemade pasta for French meals 3. Crayfish with sweetbreads 4. Three kinds of quenelles arranged on 1 plate 5. Swordfish 6. Oysters -- in pates, stuffings, sauces 7. Fresh red bell peppers as garnish 8. Vegetable pates and mousses 9. Seasonings: mustard, say, ginger, sesame 10. Garlic cooked as whole cloves 11. Puff pastry, covering soups and snails 12. Home-smoked foods: fish, meat, caviar, cheese 13. Sherbets of exotic fruits: pomegranate, avocado, passion fruit 14. Two sauces for one dish 15. Raw food: fish, lamb, beef 16. White caviar, black chocolate mousse 17. Cooked cucumbers and radishes, julienned 18. Lime or grapefruit in sauces or garnishes 19. Small pieces of fish, meat, vegetable 20. Fish wrapped in lettuce leaves 21. Nut oils: walnut, hazelnut 22. Fish with fruit 23. Pink, for sauces, quenelles, peppercorns 24. Wild mushrooms (fresh) 25. Cabbage 26. Liver -- calf, goose or duck 27. Paillards of beef, veal, duck 28. Steamed vegetable platters 29. Marrow as garnish for steak 30. Sausages made of seafood 31. Arugola 32. California wines OUTS 1. Lunch at noon, unless it lasts past 3 2. Pasta for Italian meals 3. Lobster bisque 4. One kind of quenelle on a plate 5. Dover sole 6. Oysters on the half shell 7. Parsley garnish 8. Pork pates 9. Seasonings: oregano, paprika 10. Minced garlic 11. Puff pastry wrapped around beef 12. Commercially smoked foods 13. Ice cream 14. Sauces thickened with flour 15. Overcooked fish or meat 16. Black caviar, white chocolate mousse 17. Sliced raw cucumbers and radish roses 18. Lemon or orange in sauces or garnishes 19. Single large piece of meat, fish or vegetable 20. Fish wrapped in puff pastry 21. Vegetable oils or margarine 22. Fish with tomato sauce 23. White, for sauces, quenelles 24. Cultivated mushrooms 25. String beans, unless hair-thin 26. Steak 27. Steak 28. Steak 29. Onions rings as garnish for steak 30. Sausages made of meat 31. Iceberg lettuce, except as a wrapper 32. French wines, unless they are bargains.