Cachet - Found on many First Day Covers, a cachet is text or drawings privately applied to an envelope for a commemorative purpose. Most First Day Covers use only one stamp if the stamp meets the minimum postage requirements, but often blocks of four were used. The example at right may seem a bit excessive since the going rate for mailing a letter was then three cents!

CAM - An acronym for "Contract Air Mail", CAM is mail, carried by private contractor, that was charged a fee based on the number of zones the mail had to cross.
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Cancellation - any postal marking applied to a postage stamp to prevent its reuse. This includes methods such as punching a hole in the stamp, scraping the surface of the stamp, and slicing the stamp. The term "cancellation" is not synonymous with "postmark", although they are often used interchangeably.

Canceling Machines - machines that cancel stamps and postmark envelopes in a single operation. Many different makes and models of canceling machines have been used by the various U.S. Post Offices.

Center Line Block - Many of the sheets printed for U.S. postage were separated into smaller panes before being sent to post offices. As a guide in cutting the sheets, two perpendicular guide lines were added through the center of the sheet dividing it into four equal pieces. A block of four stamps from the very center of the sheet contains the two intersecting guidelines.

Centering - the relative position of the design of a stamp in relation to its margins. For collectors of U.S. stamps with no faults, centering is of prime importance in determining value.

Certificate of Authenticity - a certificate from a recognized authority attesting to the genuineness of a philatelic item
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Certificate of Mailing - a service that provides evidence of mailing, first made available in 1915

Certified Mail - In 1955, the post office began a service that provided a mailing receipt and a delivery record, requiring the signature of the recipient.

Chads - the bits of paper removed in the perforating process. If a chad is not removed properly it results in what is known as a "blind perf". It is considered unethical and will lower the value of a stamp if the chads are removed by the collector in a discernible way such as punching them out with a perforating tool, since in a way it is destroying a bit of philatelic history.

Changeling/Color Changeling - a stamp whose color has been changed through a chemical reaction, either natural or artificial. The stamps at right show the normal brown Scott 210 along with a dramatic color changeling. 

Chase, Carrol - a great American philatelist renowned for his work on the three cent stamp of the 1851 issue

China Clay Paper - an experimental paper with an accidentally high mineral content (20% rather than the specified 2%) in yet another attempt to offset the paper shrinkage and associated waste that haunted the BEP at the turn of the 20^th century. This experiment was on the tails of the Washington Franklin "Blue Papers". The China Clays can be distinguished from the Blue Papers, both of which appear grayish from the back, by the thickness of the paper they were printed on. The China Clay stamps are much thinner and crisper. The existence of these stamps is sometimes debated and assuming they do exist they are very rare. Scott has deemed not to give this variety a separate catalog listing as with the Blue Papers.
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China Clipper - the nickname of the Martin M-130 seaplane, depicted on the three "China-Clipper" Air Mail stamps of 1935 and 1937. The China Clipper carried mail across the Pacific to and from San Francisco, with inaugural service to Manila via Honolulu and Guam on November 22, 1935. By 1937 this route had been expanded to include Hong Kong.
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Circular Date Stamp - a circular postmark generally containing the city and state of mailing as well as the date and time the postmark was applied

Classic - an early issue. "Classic" often has a connotation of rarity, but many classic stamps are decidedly common, for example the three cent stamp of the 1861 issue, Scott 65, and in fact, a scarce recent item may be properly referred to as a "modern classic".

Cleaned - Soiled or stained stamps are sometimes cleaned with chemicals to improve the appearance of the stamp. Sometimes a cancellation is removed by cleaning, making a used stamp appear unused. Unless it is done to preserve the stamp, both practices are unethical, and are easily detected by experienced collectors. Cleaning usually lowers the value of the stamp. 

Clipper - a generic name for the planes flown by Pan American on its trans-Atlantic and trans-Pacific routes.

Closing of the Mail - the time of day after which mail will no longer be accepted by a local post office, without a late fee, for dispatch to a train or ship

Coil Leader - a strip of stiff brown paper attached to the first stamp in a coil roll (the leader) 

Coil Stamps - Coil stamps are long rows of stamps, a single stamp wide, placed either end-to-end or side-by-side. Most U.S. coils have a straight edge on the two parallel sides not attached to other stamps and perforations on the two parallel sides attached to other stamps.

1˘ Washington Rotary Coils with Leader Attached

"COIL STAMPS" - a marginal marking on some of the one and two cent stamps of 1914, Scott’s 424 and 425. When the production of rotary press coil stamps proved satisfactory in 1914, the remainder of the flat plate sheets destined to become coil stamps were set aside as waste. The "COIL STAMPS" inscription was normally trimmed off the sheet during coil production, but it remained on the sheets that were set aside and later used for the regular issue. These were the first of the "coil waste" stamps, but can only be distinguished from the normal stamps, those printed on normal "non-COIL" plates, if they have part of the "COIL STAMPS" inscription or one of the 8 plate numbers in the selvage. The eight plate numbers are - on the one cent: 6581, 6582, 6585, and 6589 and on the two cent: 6568, 6570, 6571, and 6572.

Coil Waste - sheet stamps created by adding perforations to leftover sheets or pieces of sheets intended for coil stamps, as an economy measure. The block inscribed "COIL STAMPS" above right is the first of the so-called "coil-waste" stamps. 

Columbia Postal Supply Co. - The Columbia Postal Supply Company of Silver Creek, New York produced a variety of cancellation machines from around 1900 until the 1950s.

Columbians - the nickname for the first commemorative series of postage stamps, issued in 1893 to commemorate the 400^th anniversary of the landing of Columbus in the New World, but also to promote the World's Columbian Exposition held in Chicago in 1893. Although the Exposition lasted from May 1 to October 30, 1893, the ticket at right was issued for admission for just a single day, Chicago Day, October 9. 
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Comb Perforation - A method of perforation that perforates three sides of a stamp at once.  A comb perforator has a long row of pins with short rows of pins equal to the size of the stamp running at right angles to it. Each stroke of a comb perforator makes perforations in the pattern of a comb. The comb perforator need only be run in one direction, since the vertical columns of perforations, the teeth of the comb, are cut into the sheet as each horizontal row is added. Understanding the distinction between a comb perforator in which the perforations line up correctly both horizontally and vertically, and the line perforator, which almost always has overlapping horizontal and vertical perforations, may provide clues as to the authenticity of a stamp, since many early stamp were perforated using line perforators, and the perfs should not line up exactly.

Commemorative - a stamp placed on sale in limited quantities and for a limited period of time, often honoring a person, place, or event, but also used to promote certain, then current, events

Commercial Cover - a cover of apparent commercial origin, as opposed to philatelic origin, presumably the only way to be certain a stamp was used for the purpose for which it was intended and not for philatelic reasons

Compound Perforation - two different gauges of perforations used on the same stamp

Condition - the overall appearance and soundness of a stamp or cover, often the over-riding factor in the determination of the value of a stamp. Desirable "conditions" include fresh full color, a light, legible and well centered cancellation on the used stamp, full original never hinged gum on the unused stamp, sound perforations, etc. Undesirable features include creases, tears, thinned paper, short perforation teeth, and toning, among others. 

Continental Bank Note Co. - The Continental Bank Note Company held the contract to print U.S. postage stamps from 1873 to 1879. The stamps were printed on a hard white paper, helping to distinguish them from the American Bank Notes that followed. Continental added a secret mark to many of the Bank Note denominations, but not all, in order to distinguish their stamps from the National Bank Note Printings.

Contract Air Mail (CAM) - the transport of airmail by private contractors over established routes

Control Perfin -  perforations through the face of a Schermack or Mailometer privately produced coil stamp. The perforations were in the design of the initials or insignia of the company that used the stamps for mass mailing. This was done in an attempt to thwart theft of the companies stamps, since the control perfin should not be found on any other correspondence than that of the company itself. see Robert A. Siegel Auction Galleries, Sale 846. 

Convention Rate (Treaty Rate) - a special postal rate negotiated between the US and a foreign country

Cork Cancel - a canceling device made by cutting a design out of cork by or at the direction of a local postmaster. Most of these "cork cancels" are very collectible, in particular the Waterbury cancels.

Counterfeit - Although the terms are often used interchangeably, counterfeit and fake have slightly different meanings. A counterfeit stamp is designed to deceive postal authorities, i.e. a two cent stamp is counterfeited in the hope of gaining two cents worth of postage, while a two cent fake is produced in the hopes of gaining philatelic value from a collecting standpoint. The stamp at right is an "offset" counterfeit of the rotary stamp, Scott 634. The perforations are a dead giveaway to most collectors, the color is a little off, and the stamp is offset and not engraved. Yet these counterfeits fooled postal authorities for some time. The perpetrators were eventually nabbed and served jail time. 

The term counterfeit also covers altered cancellations or covers intended to be accepted by others as genuine.

Covel Coils - From about 1914-1916, the Covel Manufacturing Company of Benton Harbor, Michigan privately perforated imperforate stamps, Scott’s 314 and 408 for use on their advertising mailers. 

Cover - On this web site "cover" refers to any intact envelope, folded letter, postal card, or wrapper. The "cover" may have a stamp on it or may be "stampless" if mailed in the "stampless era". The term "cover" also applies to the front and back cardboards protecting booklet stamps, known as booklet covers.

Cracked Plate - Collectors consider a printing plate to have a "crack" if the crack shows up on the stamps printed from that plate. Cracks that get worse with progressive prints are termed "progressive" cracks and are collectible as such.

Cross Gutter Block - a block of stamps from the center of the printed sheet containing the intersection of the vertical and horizontal gutters. Very few issues were printed with the internal "gutters" wide enough to create a "cross gutter" block. They are, however, found on many of the Farley issues.

"CS" - a marking found in the top margin of the Georgia Bicentennial stamp (Oglethorpe – Scott 726). The letters "CS" indicate that the plate had been chromium plated.