S20, S30 and S40 - marginal imprints on 170 subject coil stamp rotary press plates that indicated that the outer frame lines on the stamps had been cut shallower to accommodate the new, rotary press method. Although the exact meaning of the markings is not known, the following explanation has been given by Miers and McLemore among others: The normal frame lines had a tendency to bleed when printed from the curved rotary press plates and an experiment was made with the frame lines cut 20% shallower, the "S20" plates. The frame lines still bled, and an even shallower cut was experimented with; these plates were marked "S40". This probably proved too light and another experiment with the frame lines cut 30% as deep as the normal stamp proved effective. These plates were marked "S30". We gather that since this proved effective, no further experiments were made and the marginal "S" markings were dropped on subsequent rotary plates.

SASE - an acronym for "Self-Addressed, Stamped Envelope", which is an unused envelope bearing the return address of the sender and proper postage. Before email, if a person wished to hear back from a dealer, they would need to include an SASE with their correspondence.

Savings Stamps - stamps issued by the Post Office Department that were redeemable in the form of U.S. Savings Bonds and in the form of credits to U.S. Postal Savings accounts, as well as in the form of U.S. War and Defense Bonds.

SCADS - an acronym for "Stamp Collectors Against Dodgy Sellers"; whose Internet presence since 2002 has helped educate collectors about fakes, forgeries, and other unscrupulous practices in the world of philately. 
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Schermack Mailing Machine Company -  the Schermack Mailing Machine Company of Detroit, Michigan, originally the Detroit Mailing Machine Company produced some of the early stamp vending and affixing machines from 1906-1926
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Schermack Perforations - privately applied perforations to imperforate flat plate stamps by the Schermack Mailing Machine Company, which later became the Mailometer Co., for use in its vending and affixing machines.

Schermack Control Perfin - a series of nine pin holes in U.S. postage stamps perforated by the Schermack Co. as a security device for its customers aimed at thwarting dishonest employees. Missing pin hole patterns were used as a code for each firm

Scratched Plate - A scratched plate is caused by a piece of debris "scratching" the plate during the printing process. Although the scratch may appear on the stamp, scratches such as this are quite common, after all the printing environment is not dust free, and only the most dramatic scratches are considered worthy of note. 

Second Bureau Issue - (US 300-322) the second set of regular postage stamps issued by the Bureau of Engraving and Printing, sometimes known as the "Series of 1902"

Second Day Cover - a cover placed on sale at the Philatelic Agency in Washington and postmarked the day after being issued in the "First Day" city or cities

Secret Mark - slight design modifications to the dies used to produce U.S. stamps that the National Bank Note Company turned over to the Continental Bank Note Company in 1873. The purpose of the secret marks was to distinguish Continental’s stamps from the earlier National Bank Note stamps.
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Seebeck Issues - stamps issued by various Latin American countries from 1890-1899, produced by the Hamilton Bank Note Company of New York. Hamilton’s agent Nicholas Seebeck agreed to print new issues of stamps each year at no charge, but in return retained the right to sell remainders and reprints to stamp collectors. This seemingly innocent gesture created a storm of controversy in the philatelic world, ruining Seebeck’s reputation and darkening the philatelic standing of the countries involved, a stain that has not completely lifted more than 100 years later. Ironically, many of the Seebeck issues are prized more than the contemporary issues of non-Seebeck Latin American issues.

Selvage - the portion of the paper on a sheet or pane of stamps that does not include the stamps themselves, in other words the outer edge or margin of a sheet of stamps. The selvage may include markings such as the printer’s imprint, plate numbers, etc. 

The secret mark on the 10¢ Banknote

Semi-official Air Mail Stamp - stamps produced and distributed by private firms operating air services that carried mail. The popular names for these stamps are the "Buffalo Balloon" and the "Rogers Aerial Post – Vin Fiz Flyer". 

Separated Perforations - a pair or multiple of stamps with perforations that are not entirely intact

Series - a set of stamps with a similar theme and various denominations

Sesquicentennial -  the 150th anniversary of an important historical event. Many of the U.S. commemoratives of the 1920’s and 1930’s celebrated the sesquicentennial of events that shaped the American Revolution.

Se-Tenant - two or more adjoined stamps having different colors, denominations, or designs

Set-off - the impression left when ink from a freshly printed sheet of stamps is transferred to the back of the sheet placed on top of it

7/1/71– On July 1, 1971, the 8¢ U.S. Postal Service Emblem stamp, was issued at every post office in the country, creating a specialty field for First Day Cover collectors.

Shade - a minor difference in the color of a stamp

Shanghai Overprints - a surcharged overprint on the Washington Franklin perf 11 issue of 1919, which in effect doubled the cost of purchasing the stamp in Shanghai, China if not paid in U.S. currency

Sheet - a large unit of stamps from which panes may be cut. Although commonly, but not correctly, called "sheets" by the general public as well as many collectors, it is a pane of stamps that is sold by the post office. The sheets of many classic U.S. stamps had only two panes, many of the Bureau issues had four panes, while modern sheets may have six or more panes.

Sheet-fed - the flat-plate method requires sheets to be "fed" into the press one sheet at a time, rather than on a continuous web of paper

Sheet Stamp - a stamp produced in sheet form, intended for sale as individual panes, as opposed to a stamp issued for coil or booklet purposes

Sheet Waste - Not to be confused with "coil waste", "sheet-waste" stamps were produced from remnants of the rotary press sheet stamps of the rotary one cent Washington Franklin, the rotary one cent of 1922 and the rotary two cent black Harding. One theory has it that these rare stamps were salvaged and perforated (perf 11) as an economy measure. Another and just as plausible theory is that these rarities were the result of experimentation with the perf 11 perforations on the new rotary sheets (rotary sheet stamps had been perforated 10 and were proving difficult to separate). Whatever the case, the rotary sheet waste stamps are an interesting and highly sought after lot. A census of these stamps may be found on the Siegel web site.

Shifted Transfer - Not to be confused with a "double-transfer" in which the stamp’s image is impressed twice, a "shifted transfer" is the result of too much pressure being applied to the transfer roll, resulting in a slippage and subsequent double-impression.

Short Set - an incomplete set of stamps, missing one or more of the more expensive stamps in the set. A short set of the Trans-Mississippis, the one cent through ten cent issues (US 285-290), missing the three most expensive stamps, the fifty cent, one dollar and two dollar stamps (US 291-293), is shown at right.

Siderographer (Siderography) - a skilled craftsman who made printing plates by transferring the design on the engraved dies to first a transfer roll and then the engraved plate. This job was especially difficult since each design needed to be "rocked " in at exactly the right position. Often the siderographer would punch his initials on the plate, usually in the lower left margin. An excellent guide to the siderographer, plate finisher and printer's initials, with lots of linked images, may be found at:
http://home.earthlink.net/~davinod/Initials.htm .

Sidewise Coil - a coil stamp with adjacent stamps attached at the sides. These stamps are often called "horizontal coils" and are always perforated in the vertical direction only. 

Signature Marking - a marking on the back of a stamp that provides a notation that an expert has examined the stamp and has certified its authenticity. These may be in the form of initials or a small identifying mark. The presence of this form of writing on the back of a stamp does not necessarily lower the value of the stamp, in fact it often adds to its romance if the signature is of a prominent philatelist. The practice is frowned upon today however, since there are alternate ways to document the stamp, especially through photography or scanning.

Siderographer's Initials - Clyde V. DeBinder 

Silk Paper - a type of paper containing small pieces of colored silk threads, sometimes found on the U.S. classics and Bank Notes, and used to produce certain revenue stamps

Silkote Paper - a type of very white and very smooth paper used experimentally by the Bureau of Engraving and Printing to produce a small number of the two cent Jefferson of the Liberty Series of 1954. Of the 500 panes of 100 that were originally printed, most were used for postage and, unaware of the new variety, not saved by collectors. The remaining unused stamps that were put aside are highly prized, and used examples even more so.

Single Line Watermark - a watermark used by the Bureau of Engraving and Printing that replaced the double-line USPS watermark. The single-line USPS was smaller and a little farther apart in the hopes that it would be less intrusive and affect less paper shrinkage than the double-line USPS. The single-line USPS watermark is found primarily on certain Washington Franklins issued from 1910 until about 1914, but also on postage dues of the same era and all parcel post and parcel post postage due stamps. Studying the watermarks on U.S. parcel post stamps would be of great help in learning how to identify the single-line USPS watermark, since all parcel post stamps must have the watermark.
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Sixth Bureau Issue - the sixth series of regular stamps issued by the Bureau of Engraving and Printing, otherwise known as the Liberty Series of 1954


Slabbing - a controversial technique of encasing stamps in a hermetically sealed container after authentication and grading, and that prohibits any further alteration. Slabbing is a common and accepted practice in the coin world, but has generally not been well received among philatelists. There is some debate as to whether paper deteriorates in a hermetically sealed environment and there is evidence to suggest that is the case. On the other hand, the hermetic seal can be broken (and the stamp re-expertized) at any time, and a slabbed stamp guarantees an investor that they are getting what they paid for.

Sleeper - a stamp that is under priced and may have gone unnoticed in a dealer’s inventory

Sloane, George B. - A highly respected philatelic author, his weekly column in Stamps from 1932 to 1958 was and still is widely quoted. In 1961 George T. Turner and the Bureau Issues Association published a compilation of these articles in a work entitled "Sloane’s Column". 

Slogan Postmark - a postmark that promotes or celebrates an event by words or images

Slug - an interchangeable metal "slug" containing information such as date or time

Small Banknotes - (US 219-229) stamps produced by the American Bank Note Company for the U.S. regular issues of 1890-1893. "Small" or "Baby" distinguishes these Bank Note stamps from the earlier, and larger Bank Notes (US 134-218). The Bureau of Engraving and Printing took over production of U.S. stamps in 1894, but used the same designs as the "small" Bank Notes. The Bureau’s stamps may be distinguished from American’s by the presence of triangles in the top of the design.
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Socked-on-the-nose (SON - SOTN) - (Bull’s Eye Cancel) a stamp with a clear postmark in the dead center of the stamp

Sold to the Book - an auction term meaning that the lot has been sold to a bidder not on the floor and represented by the auction firm

Souvenir Sheet (SS) - a small sheet containing one or more valid postage stamps, surrounded by a large margin with marginal inscriptions promoting the philatelic event for which it was issued

Space Filler - a damaged stamp that is used to fill the designated space in a stamp album until a better copy can be found

Special Delivery - a service that provided for delivery of an item after normal post office hours, meaning the item would be delivered that afternoon or evening rather than the next day. The first U.S. special delivery stamp was issued October 1, 1885. 

Special Handling - a service that provided an upgrade in the handling of parcels to the status of First Class mail

Special Printing - the reissue of a stamp or series of stamps

Specialist - a stamp collector who has limited his collecting area to the intensive study of the stamps and postal history of a given issue

Specimen - an overprint on stamps, not valid for postage, most often for distribution to the Universal Postal Union for identification purposes and to the philatelic press for publicity purposes

Speedy - a nickname for a Special Delivery letter

Splice - a strip of translucent paper pasted over two rotary stamps to repair a tear or to extend the length of a roll

Split Grill - a stamp showing portions of two or more grills

Stampless Cover (Letter) - a letter that does not have a postage stamp attached, often found on covers before the introduction of the postage stamp or prior to the time prepayment of postage was required

Stamp Specialist - a series of twenty books written in the 1930s and 1940s, generally distinguished by color name, containing a wealth of information about philatelic matters

Star Plates - flat plates with an imprint containing a star in the margin near the plate number. Stars were used to indicate an experimental spacing of 3mm between some of the stamps, rather than the normal 2mm spacing. The star alerted workers that the printed sheets had the unusual spacing and to make the necessary adjustments. The star plates are found on certain Washington Franklin sheets, on the two cent Lincoln of 1909 and some of the definitives of the 1922 Series.

Station cancel - a cancellation applied at a temporary postal station established for an exhibition or other special event

Stickney Press (Stickney Rotary Press) - The Stickney Press, developed by Bureau of Engraving and Printing employee Benjamin Stickney, is an intaglio, web fed, rotary press.

Stitch Watermark - a watermark comprised of straight or zigzag parallel lines caused by the stitching together of the ends of cloth aprons on which the pulp is assembled to make paper. Some sources state that stitch watermarks may be found on nearly every stamp issue.

St. Louis Bears - a nickname for the postmaster provisional stamps issued in 1846 by the postmaster of St. Louis. The design includes the Missouri Coat-of-Arms that features two large standing bears.

Stock Transfer Stamp - a revenue stamp used to pay the federal tax on transfers of stock ownership

Straight Edge - a sheet stamp from the margins with one or two sides naturally lacking perforations. Straight edges are usually not as desirable as fully perforated stamps, but as one wag put it, "at least I know my stamp has not been re-perforated."

Steel Blue - an early and desirable color variety of the 1861 twenty-four cent stamp. Often a color changeling is passed off as the "steel blue", certification is highly recommended.

Strawberry Variety (Snowball) - a flaw in the two cent U.S. Lake Placid stamp of 1932 that looks like a strawberry, or more romantically, a "snowball"

Strike - a machine or handstamp cancel on a stamp or cover

Strip - three or more unseparated stamps in a row. Many of the early Bureau issues were saved in strips of three with the plate number and imprint intact as shown at right. Later, collecting the entire block came into vogue, hence "plate blocks". 

Strippers - little "fingers" used to free the perforated sheets of stamps from the perforating pins. If ink accumulates on the strippers it may leave ink between the perforation holes of stamps.

Stuffer (Filler) - a stiff piece of cardboard placed inside a cover to provide protection and to enable a crisp and clear cancellation as the cover goes through the mail stream

Sulphuretted (similar: Oxidized) - The term applies to stamps which have become discolored or darkened due to minute amounts of sulfur in the air.

Superb - a stamp that is perfect in every way, including exceptionally large margins and mathematically perfect centering. On stamps from before 1935, superb condition implies high premiums, often at multiples of catalog value.

Supplementary Mail - mail posted after the regularly scheduled closing time and dispatched to a ship or train before it departed. Usually this required payment of a "late fee". Note that this payment is not a form of over-franking, a term which is sometimes incorrectly used for such covers.

Surcharge - an overprint that either raises or lowers the face value of a stamp or item of postal stationery.

Surface Printing (Lithography) - a stamp that was printed from the surface of the plate to the stamp paper, as opposed to intaglio, which prints from recesses in the plate, and typography, which prints from raised areas on the plate. Surface printed stamps usually have a smooth, solid look and feel.

Sweatbox - a closed box with a grill on which stamps that are stuck together are placed. Below the grill is a dampened sponge-like material that provides the humidity needed to soften the gum and allow the stamps to be separated, in an effort to leave the gum undisturbed.

Sweated gum - stamps that have been stored in conditions that are either too warm or too humid and usually in tightly packed glassines, melting the gum into a smooth, shiny condition.