This type of mail compares to U.S. Domestic first-class mail. This group consists of letters, letter packages, aerogrammes, and post cards. Letters and letter packages are items of mail containing personal handwritten or typewritten communications having the character of current correspondence. Unless prohibited by the country of destination, merchandise or other articles within the applicable weight and size limits may also be mailed at the letter rate of postage. The weight limit for letters and letter packages is four pounds.
Aerogrammes are air letter sheets which can be folded into the form of an envelope and sealed.
Tape or stickers must not be used to seal aerogrammes. Enclosures are not permitted in aerogrammes. Postcards are single cards sent without a wrapper or an envelope. These measurements apply to all countries. AO mail includes regular printed matter, books and sheet music, matter for the blind, small packets, and publishers' periodicals.