A love letter to a baking pan:

I know I've taken you for granted. I've shoved you around, yanked you from arctic cold to hellish hot, forgotten you at strangers' houses, dragged you across the country two or three times, even ignored you while I tried out younger, flashier models.

But through it all, you never let me down. You showed up at all my holiday dinners, saw me through the lean grad student years and kept me popular at potluck suppers. Over the decades, your strength and dependability never wavered. You eased my life. You know I'll love and treasure you forever.

My 9-by-13-inch Pyrex pan.

My mother gave me mine when I got married, just as she had gotten one from her mother when she got married after World War II. In the last half-century, more than 50 million 9-by-13s have been used for everything from roasting a chicken to baking brownies. For many, including me, it was the pan in which you baked your first cake. Or your first batch of lasagna.

Introduced in 1920, the 9-by-13-inch Pyrex pan eventually proved so popular, it changed the way recipes were written. It even changed the directions on the back of cake mixes. By the 1950s, a raft of new dishes was created to satisfy the millions of busy cooks who were using their 9-by-13s for easy desserts and all-in-one family meals. More than 80 years after the first one was sold, it's still Pyrex's No. 1 selling baking dish.

All of which got me wondering: Just what is it about this particular pan that cooks find so reassuring? Why is it that when we see a recipe that begins, "Grease a 9-by-13-inch pan . . . ," we immediately think, "Oh, I can make this."

And what about the size? Why 9-by-13 and not 8-by-12 or 10-by-141/2?

And is it true that we never would have had this clear, sturdy pan if it weren't for a husband and wife who, 90 years ago, got the bright idea that homemakers just might want to watch their food bubble and brown as it cooked?

The answers, while not always as clear as glass, are just as interesting.

It all began with the euphoniously named Jesse and Bessie Littleton.

Jesse T. Littleton was a physics instructor from the University of Michigan when he joined two other scientists at Corning Glass Works in Corning, N.Y., around 1912. The Corning scientists had just developed the first heat-resistant, or borosilicate, glass for railroad lanterns. The existing lanterns, which signaled the trains, had a safety problem with glass that grew hot from the flame it enclosed. When cold rain or snow fell on the hot glass, it shattered. The Corning researchers perfected a glass that would resist such heat and temperature changes.

When Littleton joined the team, he began toying with the idea of using the glass for cookware. While metal reflects heat, glass absorbs it, and Littleton had a hunch that the new glass might make a good baking dish. So he cut off the bottom of a large heat-resistant glass jar and brought it home to his wife, according to Pyrex experts Susan Tobier Rogove and Marcia Buan Steinhauer, authors of "Pyrex by Corning: A Collector's Guide" (Antique Publications, 1993).

Bessie Littleton baked a cake in the jar bottom and her husband took it to work the next day. The three scientists ate the cake (in the name of research, of course) and agreed that Littleton was on to something.

Bessie Littleton, meanwhile, decided to continue her own kitchen experiments. Using the makeshift glass container, she prepared a number of other dishes and made some remarkable findings: the cooking time was shorter than in metal or earthenware, the food did not stick to the glass so cleaning was easier, there was no residual smell or flavor of the food once the dish was cleaned, and the cook was able to watch the food cook and know when it was done. To Bessie Littleton, baking with glass was a winner.

Further research at Corning improved the formula, and in 1915 Corning introduced its new glass ovenware, dubbed Pyrex or "fire glass"(from the Greek root pyro for fire) for its ability to withstand heat. (The "ex" ending came from other Corning products, such as the nonexpansion glass for the lanterns, which was branded Nonex.) To homemakers accustomed to dark metal or earthenware baking pans and clear but fragile glassware, being able to bake in a virtually unbreakable clear glass dish was akin to a miracle.

"It was really a change of millennia," says culinary historian Daphne Derven, curator of food at the COPIA food and wine center in Napa, Calif. "Gas and electric ovens were becoming much more common in 1915, meaning women didn't have to stoke a fire, they could just turn a knob and get heat. And then you had this incredible little sparkling vessel that was so strong you could bake in it. The idea was astonishing."

From the beginning, Pyrex cookware included what was called an oblong utility dish, essentially a simple glass rectangle without handles. In 1920, the dimensions of the dish were close to today's -- 81/8-by-125/8-by-2 inches. It sold for $2.75, less than a third of today's price. Advertisements and Pyrex recipe booklets from the '20s recommended using the dish as a small roaster (hams were frequently pictured in the dish) or for making gingerbread or baked apples.

By the '30s, recipes calling for a 9-by-13-inch pan were showing up in cookbooks. "Joy of Cooking" author Irma Rombauer used it for her favorite chocolate cake in a 1936 edition, and a 1930 Better Homes and Gardens recipe for meringue-topped spice cake included the familiar words, "Pour into a greased 9-by-13-inch pan."

For nearly 30 years, the dish remained unchanged, slowly gaining in popularity behind the more traditional Pyrex pie plates and loaf pans. But after World War II ended and the country's economy began to boom, homemakers were ready for some changes.

Sharon Dupuy, brand manager for Pyrex, says home economists from Corning's test kitchen traveled around the country demonstrating recipes, introducing new products and listening to what housewives wanted. "These field representatives brought back recommendations for different shapes and sizes of pans. They saw that people were making more casseroles and roasts and that they needed handles to bring the dish to the table."

Based on postwar pamphlets and catalogues she found in Pyrex's archives and interviews with retired employees, "The ubiquitous 9-by-13-by-2-inch pan that we know today, with handles, began in 1949," Dupuy says.

The approval from the nation's homemakers was swift and positive.

"It was huge. It was such a big movement that Better Homes and Gardens created recipes to use for that size pan," says the magazine's food and entertaining editor Nancy Wall Hopkins.

Better Homes and Gardens, based in Des Moines, has been publishing its magazine since 1922, and its red-and-white gingham-covered cookbook since 1930. Both are considered a bellwether for mainstream American cooking. "We never call for anything in our recipes -- an ingredient or a pan -- unless it's saturated the marketplace," explains Hopkins.

The publication developed bar cookies and brownies for the pan in the '40s, but by the '50s and '60s, "it was tuna-noodle casserole, turkey tetrazzini, chicken and rice casserole and easy pork chops," says Hopkins. People began to identify that size pan with fast, simple recipes. "What started as the pan for dessert quickly became the main dish pan to feed the family," she says.

What also sped its popularity, adds COPIA's Derven, was the pan's appearance on the dinner table. "The clear glass made it seem cleaner. You could take it straight from the oven to the table. That had a lot to do with its acceptance."

Cake mix makers also got the message. The first Betty Crocker cake mix, a ginger cake, appeared in 1947 and required a 9-inch-square cake pan. Two years later, a mix for "party cake" called for either two square pans or one 9-by-13-inch rectangular pan.

Although Pyrex kept tweaking the size over the years -- from 9-by-14-by-2 inches in 1949, to 83/4-by-131/2-by-13/4 in 1953, to just calling it a "three-quart oblong baking dish" in 1961 -- cooks and cookbooks generally ignored these quirks and kept referring to it as a 9-by-13-by-2-inch pan. Finally, in 1987, the company stopped fiddling with increments of inches and made the pan truly 9-by-13 inches. (Today, Pyrex is no longer part of Corning, which sold its consumer housewares division in 1999 to World Kitchen, based in Reston, Va.)

To cookbook author Pam Anderson, the 9-by-13 size is simply perfect. "Did you know that six slices of cheap white bread fit exactly into a 9-by-13-inch pan for a breakfast strata? Three of those no-boil lasagna noodles running crosswise fit perfectly too," she enthuses.

Anderson, author of "The Perfect Recipe" and "Cook Smart," says the 9-by-13-by-2-inch glass pan is also the perfect depth. "A three-quart casserole holds the same volume, but the food takes a lot longer to get done. It gets overcooked on the outside while the center remains uncooked."

Anderson also notes that a 9-by-13 is just about twice the size of an 8-inch square pan "so you can double recipes easily."

Professional baker Greg Patent, author of the new "Baking in America" (Houghton Mifflin, 2002), says he owns "a whole bunch" of metal 9-by-13-inch pans, but only one of Pyrex. "That's because Pyrex is always the same, always consistent. With metal pans, there's dark metal, light metal, different weights of metal. When I bake in metal, I have to test the recipe four or five times," he says.

Patent has a special love for the 9-by-13-inch glass pan. In 1960, when he was 19, he won second prize in the junior category at the national Pillsbury Bake-Off. The winning recipe: "Apricot bars I made in a 9-by-13-inch Pyrex pan. I won $1,000 and a General Electric range and mixer for my mother."

Even pastry experts like New Yorker Dorie Greenspan, who has baked with some of the top pastry chefs in the United States and France, treasures her 9-by-13-inch glass pans. "I own six," she admits. She especially likes them for making cobblers and crisps, not only because it's fun to watch them bubbling but because the glass is nonreactive with the acid in the fruit.

When Greenspan recently moved to Paris to work on her newest cookbook, "Paris Sweets" (Broadway Books, 2002), a collection of recipes from the city's best pastry shops, a 9-by-13-inch pan was tucked into her luggage. "I wasn't sure I could find one there. French cakes are not made in that size," she says.

It was a fortunate decision. When Paris chocolatier Christian Constant gave her his recipe for a decadently rich chocolate bread pudding, she found one pan in which it was a perfect fit.

Do I even need to tell you which one that was?