What shape must flats be?

Flats must be rectangular in shape because this form allows for efficient use of space when packing and transporting mail. The rectangular shape provides practical benefits, such as fitting efficiently into standard mailboxes and automated sorting equipment, which are designed to handle items of this shape. Additionally, the uniformity and predictability of dimensions associated with rectangular flats help streamline the mailing process, ensuring that they fit together well in larger containers and making it easier for postal workers to manage large volumes of mail.

Other shapes, like circular or triangular, could create issues with sorting and delivery. The rectangular shape is a standard in the postal system, simplifying both the handling of mail and the design of mailing materials.