If the aspect ratio (length divided by height) is not within 1.3 to 2.5 (inclusive), the piece is a nonmachinable letter.

If the height (the dimension perpendicular to the length) of a letter-size mailpiece is not at least 3-1/2 inches, it is nonmailable. If the length (the dimension parallel to the address) of a letter-size mailpiece is not at least 5 inches, it is nonmailable. The placement of the address on a letter-size mailpiece (see 1.1.4 for postcards)may render a piece nonmailable or nonmachinable. See 201.9.0.ДЎ.1.3 Nonmailable and Nonmachinable Placement of Address on Letter-Size Pieces The height is the dimension perpendicular to the length. The length of a flat-size mailpiece is the longest dimension.

Postcard: For the purpose of determining mailability of a First-Class Mail priced postcard-size piece (see 1.1.4), the length is the dimension parallel to the delivery address as read the height is the dimension perpendicular to the length.

Letter-size: For the purpose of determining mailability or machinability (see 1.1.3), the length is the dimension parallel to the delivery address as read the height is the dimension perpendicular to the length.

Letter-size pieces (including Postcards).The following standard applies for these pieces:.

Then, determine length and height as follows: 1.1.2 Determining Length and HeightДetermine the processing category (see 1.1.1) based on the physical dimensions and characteristics of the mailpiece, without regard to address placement. See the physical standards for processing categories in 101 for retail (single-piece price) mail and 201 for Commercial Mail. USPS assigns each mailpiece to one of these 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.

There are five mail processing categories for mailpieces: letter, flat, machinable parcel, irregular parcel, and nonmachinable parcel.

USPS Marketing Mail Eligibility Decision TreeД¦00 Basic Standards For All Mailing Services 601 MailabilityДҐ.0 Handling, Content, and Extra Service MarkingsД¦.0 Mailing Containers-Special Types of Envelopes and PackagingД§.0 Packaging Standards for Mail Processed at Network Distribution CentersДЁ.0 Hazardous, Restricted, and Perishable MailД©.0 Written, Printed, and Graphic Matter Generally 1.0 General Standards 1.1 Determining Mail Processing Categories 1.1.1 Processing Categories.

Designing Letter and Reply Mail (PUB 25).

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