When we store baking pans in the Cooking Pleasures test kitchen cupboards, we stack them, just like everyone else does. And right now, we have one 13x9-inch pan that fits inside another 13x9-inch pan that fits inside another. But wait a minute. How can one fit inside the other if they're all 13x9-inch pans? We contacted the Cookware Manufacturers Association to get some answers.

The association, whose membership represents about 75 percent of the U.S. bakeware manufacturers, has a voluntary standard regarding actual manufacturered sizes for pans, according to Hugh Rushing, executive vice president of the association. While there's no requirement to match the standard, most manufacturers do comply. The standard states that a pan can vary by 1/4 inch on each measurement--width and length. The measurements are taken across the top of the pan and include any rims that are molded as part of the pan.

One-quarter inch doesn't sound like much, but the differences can add up. For example, pans that are labeled 13x9 inches might actually measure as little as 12 3/4 inches to as much as 13 1/4 inches in one dimension and 8 3/4 inches to 9 1/4 inches in the other. That's enough of a difference to be noticeable if you have one pan that measures 12 3/4x8 3/4 and another that's 13 1/4x9 1/4. When you factor in the possible difference in rim sizes, the actual opening size of the pan--its volume--could vary even more.

So what does this mean for cooks? Fortunately, most recipes are forgiving enough that the varying volumes of these pans shouldn't affect how your baked goods come out. But it's a good idea to watch baking times and check for doneness early, just in case your 13x9 doesn't quite measure up.