The fourth "class" of mail was established in 1879, with a single flat rate of 1 cent an ounce. This rate remained in effect until the major revision that established the parcel post system in 1913.

The establishment of the parcel post system on January 1, 1913, constituted a major extension of the postal service. It answered the popular demand for an efficient service to transmit packages. A large source of patronage was expected in the farm-to-consumer movement of food products. As matters developed, however, mercantile use of the service far overshadowed all others.

The inauguration of the parcel post system marked the end of the 4-pound weight limit on mail matter. In 1913 the weight limit was established at 11 pounds. Within a year, in response to public needs, weight limits of 50 pounds for nearer points and 20 pounds for more distant points were established. Later the weight limits went as high as 70 pounds. Size limits also_, have been extended since the inauguration of parcel post to meet the public demand for a more comprehensive service.

Because transportation costs are of relatively high importance in the transmission of packages, distance became an important factor in parcel post rates. Eight zones were established in 1913 and have served until the present time as the basis of the graduated rates for parcel post.

The law that established the parcel post system required that the postage on parcels be prepaid by distinctive parcel post stamps affixed. These stamps were not valid for postage on mail matter of other classes. But because of the annoyance and inconvenience to the public, the requirement to use the distinctive stamp was discontinued after 6 months by an order of the Postmaster General.

In 1879 the law placed printed matter other than regular newspapers and periodicals in third class, and merchandise and other mailable matter in fourth class. In 1914 miscellaneous printed matter if weighing over 4 pounds was shifted to fourth class from third; also, books if weighing over 8 ounces were placed in fourth class. In 1925, all matter not of the first or second classes weighing 8 ounces or less was placed in third class and over 8 ounces in fourth class, thus fixing the distinction between third and fourth class entirely on the basis of weight.

In 1928 a special rate was established for library books, and 10 years later by Presidential proclamation a special rate was inaugurated for other book mailings. The following year, in 1939, a special rate schedule for catalogs weighing more than 8 ounces was initiated by action of the Postmaster General. Controlled circulation publications, too, have special rates under the fourth-class category.

Fourth-class mail is the only postal service which by statutory provision must be operated on a self-supporting basis. The procedure prescribed by the Congress when establishing the parcel post system provides that the Postmaster General shall, subject to the consent of the Interstate Commerce Commission, reform the rates from time to time as may be necessary to insure the receipt of revenue from fourth-class mail sufficient to pay the cost of the service.