Serious hemline watchers, distracted momentarily by Princess Diana's 25-foot train, discovered yesterday the word from Paris is not long but short.

Yves Saint Laurent hoisted the hemline above the knees in his couture showing that took place at the exact time of the royal wedding. To the approval of such front-row guests as Ginger Rogers, Catherine Deneuve and Paloma Picasso, YSL cut off skirts for both daytime and evening above the knee and showed them with high-heeled pumps or a variation on the cowboy boot.

In a city like Washington, where most women do not fine-tune the hemlines to the call of designers, the news hardly seems earth-shattering. But the Saint Laurent offering is a suprise following the recent shift by major-league designers to long, full, soft, ethnic looks.

In fact, John Fairchild, publisher of Women's Wear Daily, calls it "a revolution." From Paris, after the showig at the Hotel Intercontinental, Fairchild said, "It is fabulous. Sleek and modern. It makes so much more sense than big skirts and babushkas and all the ethnic looks most designers are showing."

Saint Laurent opened his show with Mounia, his favorite model, wearing a big bubble-like, full-blown blouson ending in a drawstring tied over the hips. It was worn over a skinny skirt that bared the knees. The same shape appeared later in velvet for evening. The big bubble was also the shape of a taffetta dinner dress with ruffled hem. Saint Laurent also liked the shorter hem on square-shouldered suits and short coats with wide notched lapels that closed with one button.

Saint Laurent has traditionally shown his (private customer) couture collection on the last Wednesday in July at 11 in the morning. Even the royal wedding could not make the Paris designer change his good-luck slot.

But just to show he was aware of the big event in London, he seemed to pay more attention than usual to the bridal gown that has always been the way designers signal the end of their shows. Saint Laurent's bride was Mounia, who had opened the show, wearing a black tulle veil anchored by a gold crown. She was attended by a page and flower girl in wine velvet.

The bridal party walked down the runway to the sound of trumpets and choir music similiar to that of the royal wedding procession at St. Paul's Cathedral.