When any Paris fashion show drags on too long (a one-hour presentation should be the screaming limit), my confrere and I, seated side by side on those little gold chairs, embark on some highly competitive games of tick-tack-toe. We are supposedly fashion pros, and we are intellectually capable of absorbing the message that black satin trousers are ''in'' after seeing 20 pairs of practically identical black satin trousers. We don't really need to be hit on the head with two-hour shows, especially when they are often up to an hour late in starting, by which time the cane pattern of one's chair is indelibly stamped on the back of one's skirt for days to come, because no one has time to haul out the iron.

It may be hard to get into these shows - invitation cards are screened like those of a newly appointed ambassador presenting his credentials to royalty - but it's even harder to get out. Leaving before the end of any presentation does not ensure the warmest welcome the following season. And when the show is over you can't get out anyhow, because everyone is clogging the exits hoping to be the first to kiss and congratulate the designer, even if he has bored us to tears.

Yet in the final analysis it's all worth it! Each season generates constant excitement, even in the more commercial ready-to-wear showings in the spring and fall, which still attract far greater crowds than the ivory-tower couture presentations in July and January. As one Frenchman sums up the descriptions of high fashion vs. mass production: ''If you can't understand what it's all about, it's couture. If you have the faintest glimmer about anything, it's the ready-to-wear.'' Rather like what they're always saying about Deauville. ''If you can see Deauville from Le Havre, it's going to rain. If you can't see it, it's already raining.''

Even the musical accompaniments underscore the difference between the lofty realms of haute couture, presented in vast and elegant hotel ballrooms, and the ready-to-wear collections, with far too many people crammed into leaky, wind-swept tents set up in the Tuileries. The couture opts for classical music, either with piano and violin concertos echoing offstage or live quartets such as Ungaro always features. The ready-to-wear blares out the latest pop, and everyone seems to have a similar affinity for anything loud and discordant. One song, ''It's a Mad Mad World,'' swept through the ready-to-wear collections with lyrics aptly describing discrepancies in the various designers' approaches: ''The only thing you're sure of is that nothing is sure.''

Certainly the scope of mass production permits infinitely more fantasy than custom-made high fashion. But some of the top RTW creators, such as Claude Montana, Karl Lagerfeld (now on his own, having severed relations with Chloe, he is also doing the Chanel collections), and a handful of others charge prices that almost equate to made-to-order clothes.

Here's an idea of what it now costs to garb oneself from a major couture house. A gabardine suit with silk crepe blouse from Yves Saint Laurent's spring and summer couture collection amounted to 75,000 francs. (At the current rate of about 8.8 francs to the dollar, this comes to $8,523.) At Christian Dior, a simple dress and jacket ensemble was priced at 57,000 francs. You've got to be a petrodollar princess or a wealthy American (for whom the current exchange rate is so fabulous, and even so!) to afford prices like that, even if the silhouettes have that built-in chic that everyone assures you will be as good in 10 years' time as today. In a certain context, many of today's designers do achieve that timeless element, but no one has ever equaled the late Balenciaga or Chanel.

For those private clients who do dress in the couture, the new feeling of formality is the keynote. The theory is that one can ''get by'' with ready-made clothes in the daytime but that evening is the time to splurge, to throw your bonnet over the mill, and spend - on extravagant effects with lavish embroideries, sumptuous fabrics, and fur trimmings that proclaim to one and all that this is a truly super, exclusive, and expensive couture gown.

The couture has not developed such a broiling controversy over hemlines for years. It almost recalls the great days of Christian Dior when the ''ups or downs'' made headline news each season. The late creator often left the hems unfinished until the night before the opening, so as to keep the salient news a total secret. The sewing girls in the ateliers might have inadvertently leaked the tidings to some eager journalist on the prowl, so the genius who revolutionized postwar fashions in 1947 kept his entire staff in the workrooms all through the night, stitching the hemlines to his desired length.

''Long'' is quite definitely the top news this season, but as always, you can do anything you please. Both short and long seem right, and there's always that eternally classic Chanel length scaled just an inch or two below the knee. Ungaro, in his brilliant collection with so many Edwardian echoes, showed skirts at low calf lengths alternating with the surplice wrap crepe dresses baring the knees, plus a generous glimpse of one thigh. Most designers opt for long lengths , but Saint Laurent in his beautiful and totally commercial collection repeated his favorite knee-baring skirts for daytime, together with dozens of utterly graceful low-calf and ankle-length afternoon and evening dresses.

Since World War II, every time there's a furor over hemlines, many women choose the easy way out and wear pants - tailored for daytime, in soft silk or satin for evening. Black satin trousers? It's time for tick-tack-toe!

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