ASK SOMEONE who owns a pocket watch just what it "means" to him and a glaze may come over his eyes, a beatific smile light his face. He'll pull the piece out of his pocket, fondle it for a minute, and carefully dangle it toward you for your examination. (Even the plainest pocket watch is expected to draw exclamations and murmurs of admiration.) "The thing about a pocket watch," he may say, "is that it adds weight to time.

After all, a pocket watch isn't just a watch. It's a statement. It may be fashionable, it may be an elegant and practical version of Greek worry beads to be fondled and fussed with. It may become an extension of the wearer's persona, like a pipe or a beard. But it is almost never, simply, a fashionable affectation.

Now, Jim Rosenheim, whose Tiny Jewel Box offers perhaps the largest selection of secondhand pocket watches in the area (with as many as eighty to one hundred different pieces on a given day), scoffs at the "mystique" of the pocket watch.

"Heck," he says with a disdainful curl of his generous lip. "Most of the people who come in here to buy watches want them purely for the look. They don't care about heirlooms, feel, or even quality. All they want is something that looks good when they pull it out, and that keeps time."

But, Edward Hall down at Galt & Bro. Inc., which has been around just about as long as any jewelry store in Washington, disagrees.

Hall is the kind of man who not only believes in the mystique, but invests a lot in perpetuating it. He sells the Rolls Royce of a new pocket watches: Patek Philippe - thin, exquisitely balanced, impeccably designed and gleaming in an eighteen-karat gold casing. Rarely do you see a watch as beautiful as this one. Its price tag: $1250.

"This," says Hall in slightly husband, almost reverent tones, "is an heirloom. I don't care if it came out of the factory yesterday. It's an instant heirloom. This is a watch a man would be proud to own . . ."

At this stage it might be wise to proffer a short glossary of terms so you won't feel like a complete fool when the salesman unctuously asks if "M'sieur would prefer a style with hunting case or open face?"

Bow (pronounced as in arrow) - That part to which the watch chain is attached. Usually it is located over the winding knob.

Chain - The chain connects the watch to the wearer. The standard chain length is usually twelve to eighteen inches, but longer ones are available. For the gentleman whose major reason for wearing a watch is fashion, the chain is extremely important and since there are more kinds of chains available than there are watches, it's a good idea to get a knowledgeable jeweler to help you select the right one for the watch. (It would be considered outrageously declasse, for example, to put a silver chain on a white-gold watch.)

Crown - Should you ask the salesman about the little knob that winds the watch, for heaven sake don't say, "the little knob that winds the watch." Say crown. That should really impress him because most jewelry salesman don't know the term. Then you'll have the pleasure of explaining it to him.

Dust Cover - The back of the watch - the dust cover - opens to reveal the inner workings. If the watch is well made, the dust cover should fit very saugly. No looseness or imperfections around the rim. It should, in fact, be hard to open. The dust over, incidentally, is where you can discover what the case is really made of. If there is no reference printed inside to 18-karat anything, chances are excellent that it's not. It could be gold-filled, which is usually cheaper.

Fob - A much-maligned and grossly over-used word. ("Oh you have a fob, do you?" or "Wow, can I see your fob?") The dictionary defines fob as a pocket for the pocket watch. Ninety per cent of the pants sold at Britches of Georgetown today have fobs, according to co-owner Dave Pensky. Ninety per cent of the people who own pants with fobs on them refer to the pocket in question as a watch pocket, which is permissible, or as a fob pocket, which is redundant.

A fob is also the very end of the chain where the end is embellished by a small, anchoring T-bar, key for the watch, a Phi Beta Kappa key, or some other such trinket.

Hunting Case - The hunting case covers the face of the watch, ostensibly to keep the crystal from being damaged when a gentleman goes hunting. Hunting-case watches are much sought after, and are thus considerably more expensive than watches that lack the modification. In the old days, engravers really went to town on the hunting case, carving intricate art-works, many of them highly representational, on the exterior surface, and lengthy engraved messages on the inside.

Although it's virtually impossible to arrive at any accurate figures, only twenty per cent or so of all old pocket watches sold today have hunting cases, and a hunting case on a good new watch is almost unheard of because of the amount of labor involved.

Railroad - This is a term you'll encounter a lot, especially if you're interested in old watches. A salesman will say something like, "Of course, this is one of the old railroad watches," or "Would you like to see something in railroad watches?" Railroad tends to be a generic term today, although its roots are very specific. Railroad conductors and engineers used to carry these great, fat silver watches faced with large, dark, usually Arabic numerals. The watches were heavy and solid, not the kind that flinched at a sudden shock, and they usually came in silver, although gold railroad-style watches are available.

Repeater - A chime that rings out the hour, minute and, on the really complex, expensive pieces, second. It's also called a blind-man's watch. You press a buttom and the chimes ring out. Some have different tones for each segment of time, others just allow a pause between the hour and minute. Watches with repeaters are rare, expensive and hard to fix if they break. Rosenheim estimates that you could pay as much as $3000 for a repeater watch, and $200 every time the piece has to be cleaned. Fixing a broken repeater watch might cost anywhere from $200 to $600 or more.

Vest - This is the item, say fashion experts and jewelry salesmen, that has done more to affect the rise and fall of pocket watches than anything else, with the possible exception of World War II, which was responsible for the invention of the wrist watch, but that's another story. This year the vest is in again and thus so is the pocket watch.

The final consideration in the mystique of pocket watches is how they should be worn. This is an area in which you can make no mistake. Just about any way you want is permissible. The important thing is simply to make it look as if you've never been without one. You should be as relaxed with your pocket watch as you are with a favorite pair of slippers, your closest friend or your besset bound.

The traditional way is to wear it in one pocket of your vest and attach the chain to the other pocket, so it crosses your belly. If you have a paunch, this makes it look not unhealthy but, rather, dignified.

If you have a fob, or watch pocket, on your slacks, it will most likely be located either right on the waist band or immediately below it: thus you slip your watch in and allow the chain to hang in an elegant loop out of the pocket and back up to fasten to a belt loop.

Today's fashion nuts have been known to wear the watch in a breast pocket and fasten the fob through the boutonniere. One enterprising gentleman of leisure has even worn the watch in one breast pocket and attached the fob to the other, thus allowing the chain to dangle obtrusively across the jacket front.