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Dandy roll: A roller of mesh used in the manufacture of the paper which produces the watermark.

Dead country: A former stamp-issuing entity that has ceased issuing its own stamps, also the old name of an active stamp-issuing entity that has changed its name, so that the old name will no longer be used on stamps.

Definitive: Stamp issued in a large indefinite quantity and for an indefinite period, usually several years or more. Definitive stamp designs usually do not honor a specific time-dated event.

Deltiology: Picture postcard collecting.

Denomination: The face value of a stamp, usually indicated by numerals printed as part of the design.

Die cut: A form of separation usually employed on self-adhesive stamps. During processing, an edged tool (die) completely penetrates the stamp paper on all sides of the printed stamp, making the removal of the individual stamps from the liner possible. Die cuts may be straight, shaped in wavy lines to simulate perforation teeth, or take other forms.

Die: The original engraving of a stamp design, usually recess-engraved in reverse on a small flat piece of soft steel. In traditional intaglio printing, a transfer roll is made from a die and printing plates are made from impressions of the transfer roll. When more than one die is used in the production of an issue, distinctive varieties are often identifiable.

Directory markings: Postal indication of failed delivery attempt, stating the reason for failure. Examples are “Address Unknown” and “Moved.”

Dis-infected mail: In the 18th and 19th centuries when epidemics like cholera and plague ravaged large areas of Europe mail from the stricken countries was officially disinfected. The letters were either cut open and pierced or fumigated. Such mail item were ear marked with special cachets

Double impression: If the worker in charge of printing is not careful, a sheet can be put through the press twice having two clear impression on each stamp.

Double overprints: In the course of over printing a portion or whole sheet may get printing twice.

Double perforation: By mistake sometimes in certain stamp. Perforation is done twice. When the extra perforation is on the stamp design, it is treated as rare error.

Duck stamp: Popular name for the United States Migratory Bird Hunting and Conservation stamp, issued for use on hunting licenses. Each annual stamp depicts waterfowl. Also used to describe similar issues from the various states for use by hunters or for sale to collectors.

Dummy stamp: Officially produced imitation stamp used to train employees or to test automatic stamp-dispensing machines. Dummy stamps are usually blank or carry special inscriptions, blocks or other distinguishing ornamentation. They are not valid for postage, nor are they intended to reach the hands of stamp collectors. Some do by favor of postal employees.

Duplex cancel: A two-part postal marking consisting of a canceler and a postmark. The canceler voids the stamp so it cannot be reused. The postmark notes the date and place of mailing.

Duplicate: An additional copy of a stamp that one already has in a collection.