I own one hundred and forty-one hats. I did not set out to. My grandmother left me eleven, in their boxes, in 1981, and I put one on to see how it looked, and I have been buying hats at estate sales ever since, the way some women buy shoes and some men buy fishing rods. My husband calls the spare room "the millinery." He is not wrong.

This page is for the woman (or man; men's hats are another subject, and one I'd like to take up another time) who has found a wonderful old hat in a thrift shop or an attic and wants to know what it is, how old it is, how to keep it, and above all whether she can actually wear it out of the house in 1999 without feeling like a costume. She can. I do it every Sunday and most Saturdays.

Fashion never changes on January first of a new decade, and hats least of all; a woman wore her good hat for years. But the shapes do sort themselves out roughly like this.

The 1920s: the cloche. A deep, bell-shaped hat (the word is French for "bell") pulled down to the eyebrows, worn with the bobbed hair that was the other great shock of the decade. It was usually felt for winter and straw for summer, close to the head, with very little brim or a small one turned down, and the trim, if any, was flat: a ribbon, a small ornament, a bit of embroidery. Nothing stuck up. The cloche only works if it sits low; a cloche worn on the back of the head is a bowl. Since the hair had to be short and flat to go under it, the cloche and the bob kept each other in business.

The 1930s: the hat gets smaller and tilts. The brim comes back in the early thirties, often wide and dipping, and then by the middle of the decade hats begin to shrink and to slide forward and sideways on the head at angles that look, to us, faintly drunken. Berets, little felt hats with one feather, the "Empress Eugenie" hat with its plume, turbans, and by the end of the decade some very odd small shapes perched over one eye. Elastic cords behind the head begin to appear to hold these on, and so do hat pins in numbers.

The 1940s: the tilt hat, and making do. The war made fabric scarce and put women in uniforms and factories, and hats went two ways at once. For work and everyday there were snoods (a net bag for the hair at the back), turbans and scarves; for dressing up there was the tilt hat (or "doll hat," or "toy hat"): a tiny confection, sometimes no bigger than a saucer, worn far forward over the forehead and anchored with a band or elastic around the back of the head, often topped with a veil, flowers or a great deal of feather for its size. Hair was long and rolled up beneath it. After the war the brims came back, wide and flat, to go with the full skirts of the "New Look."

The 1950s: the last great decade. Everything at once, because everyone still wore a hat. Small close-fitting caps and "half hats" that curved over the crown with the back of the head open; "whimsies" that were hardly more than a veil on a headband, for cocktails and luncheons; the coolie and the lampshade and the mushroom (all roughly what they sound like); the big picture hat for garden parties; and toward the end of the decade a small, deep, round shape sitting on the back of the head, made for the newly enormous hairdos.

The early 1960s: the pillbox. A small, flat-topped, straight-sided hat with no brim at all, worn on the crown or the back of the head. The shape had been around for decades, but it was Mrs. Kennedy who made it the hat of the moment from the inauguration of 1961 onward, and for a few years every woman in America owned one in a color to match her suit. It is the last hat that absolutely everyone wore. By the middle of the decade the hair was too big for it, and by the end of the decade the hair had won, and the hat, as a thing a woman put on to go out of doors, was over.

Turn it over. Almost everything you need to know is inside.

The label. A label with a department store's name is a gift: many of those stores are long gone or long since merged, and a little library work on when the store existed (and under which of its several names) brackets the hat neatly. A milliner's own label, with a shop name and a city, means the hat was made or trimmed by hand in that shop, and the style of the label itself, its printing, its script, whether it's woven or printed, tells you a good deal once you've seen a few hundred. A designer's name is fun but be careful, since good names were licensed and copied freely. And two things nobody thinks of: if the label shows a ZIP code, the hat is after 1963, when ZIP codes came in, and if it uses the two-letter state abbreviation ("NY" rather than "N.Y."), that's the same period or later. A label with a union mark (the hatters' and milliners' union had its own) also helps, since the designs of those marks changed over the years.

The construction. Look at what it's made on. A stiffened net foundation (buckram) with a wire in the edge of the brim is the standard structure for a "made" hat from the twenties well into the sixties; a hat that is all one piece of moulded felt or straw with no foundation is either very cheap, very good, or later. The sweatband inside is usually grosgrain ribbon (a ribbed silk or rayon; later nylon), and its width, its stitching (hand or machine) and whether it's been replaced are all clues. A size stamped or written inside (21, 22, 22 1/2, meaning inches around the head) tends to mean a better hat from before the mid-fifties, when so many hats went to one-size "fits all" with an elastic. An elastic cord or a comb sewn in points at the thirties or after. Hat pin holes in a lining, with no elastic, point earlier.

The trim. Real feathers and real flowers (silk, velvet, sometimes wax) are older; plastic flowers and nylon veiling are later. Veiling in general is later thirties through the fifties. A great deal of veiling, in a cage over the whole face, is fifties.

Common sense. The shape tells you the decade; the label tells you the store; the wear tells you how much she loved it. And when all three disagree, you have a hat that was retrimmed at home to keep up with the fashions, which is how most women managed and which makes the hat, to me, rather more interesting than one that wasn't.

• Hat boxes. Round ones, one hat to a box if you can manage it, and never stack hats inside each other. Old boxes are fine if they're clean and dry. A box keeps out light, dust and the cat.
• Tissue. Stuff the crown lightly with acid-free tissue paper so it holds its shape, and lay tissue under a brim so it doesn't rest on its edge and go wavy. Ordinary tissue turns yellow and acid over the years and marks pale straw and silk; the good kind comes from art-supply and archival-supply houses and is worth it.
• No plastic. Not bags, not bins, not dry-cleaner's film. Plastic holds moisture and the hat mildews or the felt sags, and some old plastics give off fumes that yellow fabric. Cardboard breathes. Let it.
• Keep them cool, dry and dark. Not the attic (heat), not the basement (damp), not on a stand in a sunny window (fading, in months). A closet shelf in a room people live in is about right.
• Moths eat felt and feathers with pleasure. Cedar blocks or lavender in the box; and any new arrival goes in a sealed bag in the freezer for a week before it joins the others, to kill anything it brought along. This is the one time plastic is allowed.
• Cleaning. A soft brush, always in the direction of the nap, for felt. Steam from a kettle, held well away, will relax a dent in felt or straw so you can coax it back by hand. Nothing wet on straw. Nothing at all on velvet but a brush and prayer. Stains are a job for someone who cleans theatrical costumes, and sometimes not even them; an old stain is often part of the hat now.
• Handling. By the brim, or by the crown with clean dry hands, never by the trim. The trim is the first thing to go and the last thing that can be replaced.

Estate sales, first and best, especially the ones run by the family rather than a dealer, and especially when the lady of the house was over eighty. Go to the bedroom closet, look on the top shelf, and ask about "the hat boxes" by name; there are often more in the attic that nobody thought worth bringing down. Be kind. Somebody's mother wore those.

Church rummage sales, where hats are usually a dollar and the ladies running the table will tell you who wore it and to what.

Thrift stores, though the good ones are picked over by dealers now, and antique malls, where you will pay for the dealer's gasoline. Prices in the malls have gone up steadily in the last few years; the hats haven't changed, only the number of people who want them.

My own family. Ask your aunts. Every family over a certain age has a hat somewhere that nobody has the heart to throw out and nobody has the nerve to wear. Volunteer.

And what I look for: condition first, always; then a label; then whether it fits or can be made to (most hats before the fifties run small by today's heads, and a hat can be stretched a little but not much). I no longer buy a hat just because it's old. I buy a hat because I would wear it, or because someone once did, extraordinarily well.

You will feel silly for the first four minutes. This is the fee. After that, the following:

1. Wear one hat, all the way. A hat put on apologetically, pushed back off the face, worn with a shrug, looks like an accident. Put it on the way it was meant to be worn (a cloche low, a tilt hat forward and tilted, a pillbox squarely back) and then forget it's there. It is the shrug that looks silly, never the hat.
2. The hair matters more than the hat. Nearly every hat before 1960 was designed to sit on hair that was set, pinned, or otherwise arranged to receive it. Smooth your hair, pin it, or at least decide where it is going. A hat on top of loose hair going in all directions has no chance.
3. Match the occasion, not the decade. Do not wear a 1940s tilt hat with a 1940s suit and 1940s shoes and 1940s lipstick; that's a costume, and people will ask what play you're in. Wear the tilt hat with a plain dark dress you already own. One vintage thing, worn with conviction, looks like style. Five look like Halloween.
4. Start where hats are still welcome. Church. A wedding. The races. Easter, of course. A garden party if you can find one. A funeral, for which a small dark hat with a bit of veil is not only permitted but still, in some circles, correct. Then a nice lunch. Then, eventually, the grocery store, where you will find that the checkout girl says "I love your hat" and means it.
5. When someone asks, say "it's vintage." Not "oh, this old thing," not a story about the estate sale, not the decade. "It's vintage." Then change the subject. Half the elegance of a hat is not explaining it.
6. Expect to be looked at, and understand that in 1999 a woman in a hat is looked at with interest and a kind of wistfulness, not with disapproval. People are pleased. They're only sorry they didn't think of it.

People like to blame President Kennedy, who was often photographed bareheaded, for killing the men's hat. Perhaps. I think the women's hat died of several things at once, and none of them was a president.

The hair. When the hairdresser began to charge more than the milliner, and the result was too large and too fragile to put anything on top of, the hat lost the argument. This was well under way by 1963.

The car. A hat is for walking, for the bus, for the streetcar, for the church steps. It is not for a low roofline and a headrest, and by the sixties everyone drove everywhere.

The end of "dressed." There used to be a difference between how one looked at home and how one looked in public, and the hat was the signal that a woman was in public and to be treated accordingly. When the difference went, so did the signal. I don't entirely mourn the rules; I do mourn the hats.

The young. Every generation refuses its mother's clothes. The mothers of 1965 wore hats.

And so the boxes went to the attic, and thirty years later they come down to the estate sale where I'm waiting, cash in hand, at eight in the morning, in a hat.