Mrs. Emily Post, for many years a leading authority on all matters of social good form, died Sunday in her apartment at 39 East Seventy-ninth Street after a brief illness. Her age was 86. In a time when many believed that good manners were necessarily elaborate, Mrs. Post was a pioneer in simplifying them. If you picked up the wrong fork, there was a possibility that your hostess was wrong in having too many forks, Mrs. Post taught. It wasn't necessarily you who were wrong. She believed that what was socially right was what was socially simple and unaffected. She taught as the basis of all correct deportment that "no one should do anything that can either annoy or offend the sensibilities of others." Thousands found their social problems solved by her simple counsels. Her name became synonymous with good manners. Mrs. Post's advice was varied. She gave suggestions about how to inculcate good manners in an active 7-year-old boy and she could and did answer complicated questions about the proper way to address titled persons of Europe. But for the most part she advised the debutante, the confused suitor and the newly married couple who wished to establish themselves in good relations with the world about them. She always avoided giving lonelyhearts advice and never suggested ways to capture a husband or wife, although many young persons found courtship easier because of what she said. When Mrs. Post's first widely known book, "Etiquette," was published in 1922, many of the existing handbooks of social usage presupposed wealth and elegance on the part of the reader. Mrs. Post offered advice to society as it was and answered basic problems that confront ordinary people who want to have good manners. Every edition of her book emphasized the basic rule of etiquette: make the other person comfortable. The book was revised ten times and was in its eighty-ninth printing. For many years Mrs. Post had a radio program and a daily column, which is still being syndicated to more than 200 newspapers. Her mail was enormous and she had to set up a special office to handle it. It was largely due to Mrs. Post's gentle guidance in the modern frame of mind that the chaperone tended to become a less important feature of society. The duties of the chapeone and, indeed, the very chaperone herself, tended to disappear from later editions of Mrs. Post's book. However, when a bachelor girl asked her if it was correct to visit a man alone in his apartment, she always said no. Mrs. Post never made capital out of her own social position, which was excellent. She had first-hand knowledge of the social world at Bar Harbor, Me., where she spent her summers, and she traveled extensively in Europe. In addition to establishing herself as a leading authority on good manners, Mrs. Post enjoyed some success as a fiction writer. She wrote a novel, "The Title Market," and her short stories as well as her articles on good social usage appeared in such magazines as Scribners and Colliers. She even wrote a cook book, which was widely sold under the title, "The Emily Post Cook Book." Other books by Mrs. Post include "The Personality of a House," "Children Are People," "How to Behave Though a Debutante," "Purple and Fine Linen," "By Motor to the Golden Gate" and "The Eagle's Feather." Her first book, "Flight of the Moth," appeared in 1904. In 1946 Mrs. Post founded the Emily Post Institute, where problems of gracious living were to be studied. Mrs. Post trained the staff herself. Born in Baltimore in 1873, Mrs. Post was a daughter of Bruce and Josephine Lee Price. Her father was an architect. She was educated privately at home and atttended a finishing school in New York, where her family moved when she was 10. Mrs. Post was about 5 feet 9 inches tall and had soft gray hair and blue eyes. Her pet dislikes were pretentious people, dirty silver and hostesses who served themselves first. She liked terrapin, boiled hominy and strawberry and could tell how they should be cooked and served and when each was appropriate in a meal. For many years she spent part of her summers in a 170-year-old house in Edgartown, Martha's Vineyard, Mass. She designed the decorations of the house herself and supervised its remodeling. Last year Mrs. Post wrote a series of columns about motor manners. She had long been appalled by the thoughtlessness of persons driving in traffic and sought to set down a few simple maxims of good manners to take the scowl and snarl out of automobile driving. Mrs. Post was the center of a notable contretemps when she spilled a spoonful of berries at a dinner of the Gourmet Society here in 1938. With faultless etiquette, the ninety-two members of the society pretended not to notice, and continued eating. The press, perhaps a bit less mannerly, made the most of the little faux pas, but Mrs. Post laughed it off. Mrs. Post seldom hesitated to answer a question in her field and often she simply applied common sense to the problem. Once she was called upon to settle the question: Is dunking doughnuts correct?" She answered: "Any place that would have doughnuts would be like a picnic, where, short of smearing wet doughuts from ear to ear, you could do pretty much as you pleased. You wouldn't have doughnuts at a formal dinner anyway." Elbows on the table while eating? Mrs. Post said: "The rule is against elbows on the table while eating. If not using the arm as a lever swinging a fork or spoon from plate to mouth, it really doesn't make much difference." "Etiquette," she once said, "is the science of living. It embraces everything. It is the code of sportsmanship and of honor. It is ethics." In 1892 Emily Price was married to Edwin Main Post, a banker, member of a family widely known in social circles on Long Island. This marriage was terminated by divorce. Mr. Post drowned in a sailboat accident in Fire Island Inlet. Two sons, Edwin M. Post Jr. and Bruce Price Post were born to this marriage. Bruce Price Post died in 1927. A grandson, William Goadby Post, also survives. A funeral service will be held Thursday at 10.30 A.M. in St. James Protestant Episcopal Church, Madison Avenue at Seventy-first Street. Burial will be private.