Pieces exceeding the maximum dimensions of a letter may qualify as what?

When pieces exceed the maximum dimensions established for letters, they usually qualify as flats. Flats are defined as mail that is larger than letters but smaller than parcels, typically involving items like catalogs or large envelopes that can still be delivered without the dimensional requirements that parcels must meet. The classification as a flat allows for different postage rates and handling processes that are adapted to the size and thickness of the mail piece.

Flats must adhere to specific size measurements, being larger than the maximum for letters but not exceeding the limits for parcels. This classification is significant for postal operations as it helps ensure that mail is handled appropriately and efficiently, aligning with pricing structures that reflect the physical characteristics of the mail piece.