“A” Marginal Imprint - The letter “A” was used in the margins of some of the Washington Franklin plates to indicate uniform vertical spacing of 2.75mm between the rows of stamps. The early Bureau Issues, as well as the earlier Washington Franklin stamps, were printed with 2.0mm spacing. In 1909 an experiment was made to reduce some of the waste after the stamps were perforated, waste estimated as high as 20%. Since the outer edges shrunk more than the center of the sheet when the paper dried, it was decided to increase the vertical spacing in the outer rows only, to 3.0mm. Only a few plates had this combination of vertical spacing and they were given a small solid star to indicate the new spacings. The experiment was considered a partial success and prompted a new plate layout with 2.75mm vertical spacing, the “A” plate. This new spacing proved so successful that it was adopted across the board for later issues and the “A” notation was dropped.

AEF Booklet Panes - (Scott's 498f and 499f) Booklet panes of 30 of the one and two cent Washingtons of 1917, made explicitly for use of the American Expeditionary Forces of the Army serving in Europe in World War I. These booklet panes are quite desirable, since they were only issued for a few months and not widely collected at the time. It is estimated that less than 200 of the two-cent pane, Scott 499f, exist. 
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Affixing Machine - A machine that affixes stamps to an envelope, card or wrapper automatically, resulting in dramatic time savings for mass mailers more...

Air Mail - mail that is carried by airplane or other airship, such as a dirigible

Air Mail Special Delivery Stamp - (Scott's CE1 and CE2) a stamp that pays the fee for both airmail and special delivery. Although the first two airmail stamps of 1918, Scott’s C2 and C3 also provided for special delivery service, they are not generally included in the category of Air Mail Special Delivery Stamps. All other Air Mail stamps, other than Scott C2 and Scott C3, did not include Special Delivery as part of the Air Mail service.  
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Air Mail Stamp - a stamp paying the fee for airmail service. The U.S. issued its first Air Mail stamps in 1918 and discontinued stamps for domestic Air Mail service in 1977. Since 1977 Air Mail stamps are issued only for international airmail.
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American Bank Note Co. - The American Bank Note Company held the contract to print U.S. postage stamps from 1879 to 1894. The printings were made on a soft porous paper, helping to distinguish them from the other “Bank Notes” of the 1870s. Additionally, they printed the Overrun Nations issue of 1943-44, and a few others since 1979.  

American Guideline Society - the predecessor organization of the United States Stamp Society

American Philatelic Expertizing Service - a service of the APS that renders authoritative opinions on the genuineness of stamps and covers
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American Philatelic Research Library (APRL) - Often overlooked by mainstream collectors, it is the largest private philatelic library in the United States, with extensive resources for researchers and collectors. 
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American Philatelic Society - the largest philatelic organization in the United States
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American Postal Machine Company - a major manufacturer of canceling machines which were used from the 1880s to the 1940s

Aniline Inks - Substandard inks made with synthetic pigments used in lieu of the normal imported organic inks, primarily from Germany, when they became unavailable due to the First World War. The synthetic inks had a tendency to bleed through the stamp paper giving a pinkish hue to the back of the stamp. These “pink backs”  are found primarily on the two, three and twelve cent perf 10 single-line watermarked stamps, Scott’s 425, 426 and 435a.  

APO - the acronym for "Army Post Office"

APRL - the acronym for "American Philatelic Research Library"

APS - the acronym for "American Philatelic Society" 

Army Post Office - (APO) A US post office with a uniquely assigned number, established to process mail for overseas military units

Armstrong, Martin A. - The author of several important U.S. philatelic references, including: United States Coil Issues 1906-1938; “Washington Franklins 1908-1921”; and U.S. Definitive Series 1922-1938

Arrow - A marginal marking, shaped like the tip of an arrow which served as a guide for cutting sheets into smaller panes, and to guide the perforation process

Arrow Block - A margin block with a guide “arrow”

Ashbrook, Stanley - Universally regarded as one of the greatest American philatelists. His two-volume study of the one cent stamp of 1851 is considered the definitive work on the subject.

Atherton Shift - a remarkable double transfer “shift” in the upper left corner, particularly in the numeral “2”, of the 1861 2¢ black Andrew Jackson, nicknamed the “Black Jack”
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Attleboro Perforations - Privately applied perforations to imperforate stamps in 1909 by The Attleboro Stamp Company for use in their affixing machine. The stamps were used to send the company’s newsletter, the Attleboro Philatelist.
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