What distinguishes a letter from a flat item in postal terms?

In postal terminology, the distinction between a letter and a flat item is primarily based on size and shape. Letters are typically defined as items that fall within standard dimensions, generally considered to be rectangular pieces of paper, such as standard envelopes containing documents or cards. Flats, on the other hand, are larger than the minimum size for letters and include items like catalogs, magazines, or larger envelopes that exceed typical letter dimensions.

This size classification is crucial because it impacts how items are processed, sorted, and transported within the postal system. Understanding the difference helps postal workers adhere to regulations regarding mailing standards, ensuring proper rates and handling. While there are specific physical characteristics associated with weight and handling, these do not fundamentally differentiate between letters and flats as effectively as their size does.