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(Upper left) the upper left pane from a sheet containing four panes. "UL" is also used to describe plate blocks taken from the upper left corner of such sheets.

Ultraviolet (UV)

A wavelength of light just beyond the visible spectrum used to detect luminescence on tagged stamps and certain flaws on older stamps. There are two ends of the UV spectrum used to detect luminescence, short wave and long wave. Short wave is used to detect phosphor tagging and long wave to detect fluorescent tagging.


A stamp booklet that is in the state in which it was sold by the post office, that is, not taken apart.

Ungummed As Issued

Stamps issued without gum, as opposed to stamps from which the gum has been removed. Many of the U.S. reprints and special printings were issued without gum.

United States Post Office Department (USPOD)

The branch of the government responsible for the operation of the Post Office. Upon its reorganization in 1971, the name was changed to the "United States Postal Service" (USPS).

The United States Specialist

The monthly journal of the United States Stamp Society, formerly known as the "Bureau Specialist". The U.S. Specialist is a veritable fountain of information for the U.S. collector.

Universal Postal Union (UPU)

An organization, founded in Berne, Switzerland in 1874 to regulate and standardize international postal usages and rates.

Universal Stamping Machine Company

A major manufacturer of canceling machines from the 1900s to the 1990s.


A stamp or other philatelic item not recognized by the catalog publishers.


A stamp printed without tagging. If this lack of tagging was unintentional the stamp is an error stamp, in the fullest sense a "missing color" error; it is simply missing a "color" that our eyes can t see.


A stamp that has not been used for postage and therefore has no postmark. The stamp may have only partial or even no gum or may have been hinged so as to no longer be regarded as "mint". The terms "unused" and "mint" are often used interchangeably, but many would like to see the term "mint" reserved for stamps that have full, unhinged gum with bright and fresh color; stamps that are in the same condition as when they left the post office. A stamp may have gum that is not pristine or a lack of fresh and vibrant color, or any other fault for that matter and yet the stamp is still "unused" if it did not pass through the mails and does not have a cancellation.


Stamps printed on paper without watermark, alternately, a stamp showing no watermark.

U.S. Automatic Vending Machine Company Perforations

Perforations privately applied to imperforate stamps by the U.S. Automatic Vending Machine Company of New York for use in its vending machines, from 1906-1912.


A stamp with either postal or revenue usage with a cancellation of some sort to note the fact.


(United States Internal Revenue) A double-line watermark found on some revenue stamps. The USIR paper was inadvertently used to print three ordinary U.S. postage stamps, the six and eight cent stamps of the 1895 issue, US 271USIR and US 272USIR, and the one dollar stamp of the 1938 issue.


(United States Postal Service) An independent, self-supporting federal agency formed to run the Post Office, established on July 1, 1971, by the Postal Reorganization Act.

U.S. Specialist

The monthly journal of the United States Stamp Society, formerly known as the "Bureau Specialist" (see United States Specialist above).