Sample - a stamp produced by security printers to demonstrate the printing capabilities of their firms. Samples employed actual stamp designs in different colors, were usually overprinted or perfined to denote status, and were given to prospective clients at trade shows and philatelic exhibitions.

Scratched plate - a blemish on the plate caused by contact with a hard foreign object

Screen wagons - wagons built specifically to deliver mail. They were first used in 1886.

Se-tenant - stamps or labels printed from the same plate that adjoin one another but have different designs and colors

Sea Post Service - bona fide post offices that operated on ships traveling regular routes, primarily between 1891 and the 1930s

Self-service registration stamp - a stamp with the dual purpose of registration and special handling for use with letter packets and parcels. The stamps were issued in pairs from automatic vending machines with a certificate of posting. One stamp was affixed to the package; the other to the certificate as sender's evidence. They were used primarily in the German Democratic Republic, 1967-1968.

Semi-official envelopes - imprinted envelopes provided by the Confederate Government. Not everyone was granted franking privileges, however. 'Semi-official' envelopes, like the example shown here, required prepayment of postage in stamps.

Semi-official stamp - a stamp used in connection with private postal services but having official sanction. Examples include England's railway letter fee stamps and the U.S. Buffalo Balloon covers, which were delivered by a private balloon to the nearest post office, where franking continued through the regular postal service.

Semi-postal stamp - a postage stamp bearing a higher-than-normal postage rate. The excess revenue was given to charity or some other cause.

Separation - a general term applied to any means provided for separating stamps. The earliest and some more recent issues had no provision for this. Such stamps are usually called 'imperforate'. Many methods have been developed for removing individual stamps from sheets. These processes are termed 'perforating' and 'rouletting'.

Serpentine roulette - a form of roulette consisting of wavy lines

Serrate roulette - a zig-zag line of short dashes

Sheet number - colored numbers printed consecutively on sheets of stamps which identify individual sheets as well the number of sheets printed

Silver tax stamp - a revenue stamp used to pay a tax on profits from transactions in silver

Sinking fund stamp - a stamp used for postage in France with a premium for the reduction of the national debt

Slogan - a postmark containing a message or announcement

Soldiers' stamp - a stamp issued to exempt servicemen's mail from postage

Souvenir sheet - a special postal issue, typically used to commemorate special events, which incorporates one or more stamps. It is produced on small format paper with wide margins, often printed with a unifying background and/or title. The stamps may or may not be perforated and usually differ in design and possibly denomination from each other.

Special delivery issue - a stamp that pays for special handling and accelerated delivery of mail, usually by a courier. Expedited services, such as Express Mail, have largely replaced the need for special delivery.

Special Delivery Service - a service authorized on March 3, 1885, which provided prompt delivery of all letters affixed with a 10-cent special-delivery stamp. The service extended to free-delivery offices in towns of 4,000 or more inhabitants. By August 4, 1886, every free-delivery office, including Rural Mail Service, was equipped to handle special-delivery letters.

Special flight stamp - stamp for use on mail carried on special flights. Usage: Netherlands - inscribed Bijzondere Vluchten; Switzerland - inscribed Pro Aero. PTI, 1987.
Special handling stamp - a stamp supplementing the fourth-class postage rate so that fourth-class mail was handled as first-class

Square perforations - small, square holes, which are quite rough in appearance

Stamp classifications - a system by which all stamps are organized and defined according to their functions or type of authority controlling their issue

Stamped envelope production - a technique using a resilient plate instead of a rigid one. This technique allows the paper to be forced into the depressions of the die to show as a colorless relief on the finished stamp.

Star route - a mail route, usually rural, served by a private contractor. The routes pre-dated RFD (Rural Free Delivery). The postmaster general was allowed by Congress to form contracts with private delivery services without specifying the mode used for the deliveries. To identify these routes, an asterisk was placed on the Post Office Department records. Consequently, they became known as 'star routes'.

Stock transfer stamp - a revenue stamp tax on stock transfers and certificates which was used by federal and state authorities from 1918 until 1952

Sunday delivery stamp - a stamp used on mail delivered on Sundays and public holidays, the revenue from which was used to maintain a sanatorium and rest homes for postal employees and their families. The Sunday delivery stamp was used in Bulgaria, 1925-1929, and in 1942.

Surcharge - an overprint on the face of a stamp which changes, adds to, or confirms its face value