The Postal Service has five mail processing categories for mailpieces:

  1. Letters.
  2. Flats.
  3. Machinable parcels.
  4. Irregular parcels.
  5. Nonmachinable parcels.

The Postal Service assigns each mailpiece to one of these mail processing categories based on the physical dimensions and characteristics of the mailpiece using the longest dimension as the length, regardless of the placement or orientation of the delivery address on the piece. USPS standards exist so that mailers can design mailpieces that the Postal Service can effectively process on our sorting machines.

This publication is intended to provide guidance for mailers who design letter-size mailpieces, and especially automation-compatible designs. Printers, graphic artists, forms designers, envelope manufacturers, and computer programmers can find useful information from basic addressing to technical specifications.

This publication is based on Mailing Standards of the United States Postal Service, Domestic Mail Manual (DMM), which is the final authority on domestic mailing standards. The Postal Service routinely updates the DMM.

The DMM contains the standards governing domestic mailing services and is available on the Postal Explorer Web site at If you are not familiar with the organization and numbering system in the DMM, a tutorial is available on the Postal Explorer Web site.

Notice 123, Price List, is the published list of USPS prices and is available on the Postal Explorer Web site at

Some of the information in this publication is technical. For additional assistance, contact your local Post Office.