II.

Classifications of Postal Matter

Mailable matter in the domestic service is divided into four classes, for which different rates are charged. First-class mail includes letters, postcards, matter wholly or partly in writing, and matter sealed or closed against inspection; second-class mail comprises newspapers and periodical publications; third-class mail (less than 16 oz/170 g) includes books, circulars, matter wholly in print, and proof sheets; and fourth-class mail (domestic parcel post, 16 oz or over) covers merchandise and all matter not covered in the other three classes. Express mail, the newest service, provides overnight delivery for packages of up to 70 lb. Letters and postcards sent by airmail to foreign countries are considered first-class mail, as are parcels sent by air or as registered mail. Additional fees are charged for special delivery or special handling. No airmail category exists for first-class letters within the United States.