Our food editor did some digging to find out.
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I won't keep you in suspense. There is no significant difference between a13- by 9-inch cake pan and a 9- by 13-inch cake pan. They are the same thing. Yes, you read that correctly.

Pans made by different manufacturers may vary slightly by depth—2 inches versus 2 ¼ inches, for example. But whether the pan is labeled as 13- by 9-inch or 9- by 13-inch, it should have a volume of about 14 cups (or 3.3 liters) and be the right size for most sheet cake or bar cookie recipes. (If you want to measure your pan just to make sure, measure it from the inside, from one corner to the opposite corner on the same side of the pan.)

So why can't we agree on what to call this pan? I reached out to a few major cookware brands to find out why they choose one measurement over the other. The answers revealed what I figured was true: anything goes!

Although specific sizes and shapes of pans are common (like Bundt pans, loaf pans, and round cake pans) there is no standard for their measurements. Companies can set their own standards for measuring pan sizes, which creates confusion for shoppers.

Williams-Sonoma, a giant in the bakeware world, said the company lists the width first, then the length, so 13- by 9-inch. A spokesperson for USA Pan, a manufacturer of aluminized steel pans, said that the company lists the greater dimension first for all of their pans (such as a 10- by 5- by 3-inch loaf pan). King Arthur Flour sells 9- by 13-inch pans, but for loaf pans or other rectangular pans, length comes before width. In Southern Living food stories, we refer to "13- by 9-inch" pans—but a look through our archives proves that even we've slipped up a few times.

My personal theory is that you call the pan by the name you grew up hearing. For some people, saying the smaller number first just makes sense. For others, length always comes first. Either way, you're not wrong—as long as you agree that both pans are the same.