U - prefix letter used by U.S. Bank Note Company in front of the plate number on its modern stamp production.

U.S. Automatic Vending Co. - New York based manufacturer of vending machines and private coils.

U.S. Automatic Vending Perforations - perforations privately applied to imperforate stamps by the U.S. Automatic Vending Machine Company for use in its vending machines.

U.S. Bank Note Co. - contractor for postage stamps beginning in 1989.

U.S. Official Postal Guide - primary resource for information on post office listings, domestic and foreign postage rates, policies, orders and regulations.   Published from 1890 to 1953.

UGAI - see Ungummed As Issued.

UL - Upper left.  Refers to the upper left pane from a traditional sheet of four panes.  These initials are also used to describe plate blocks taken from the upper left corner of such sheets.

Ultraviolet Light - light beyond the visible spectrum used to detect luminescence.  Long wave ultraviolet light is commonly used to detect fluorescent brighteners, while short wave is used to detect phosphor tagging.

Uncoated Paper - Paper with a rough surface, with many jagged "hills and valleys" when looked at under magnification.  See also  Coated Paper and Prephosphored Paper.

Underfranked - an item of mail bearing insufficient postage.

Unexploded - a stamp  booklet that is intact, that is, not taken apart.

Ungummed As Issued - stamps issued without gum, as opposed to stamps from which the gum has been removed.  Farley's Follies were issued without gum.

United States Envelope Company - manufacturer of modern stamped envelopes.

*United States Post Office Department - the branch of government that operated the Post Office prior to its reorganization in 1971.

*United States Postal Service - the government corporation formed to run the Post Office in 1971.

The United States Specialist - monthly journal of the United States Stamp Society.

United States Specialist - monthly journal of the United States Stamp Society.

*United States Stamp Society - leading organization of collectors of U.S. stamps, formerly known as the BIA (Bureau Issues Association).

Universal Postal Union - an international organization, tracing its origin to 1863, which facilitates the transmission of mail between member countries.

Universal Stamping Machine Co. - major manufacturer of cancelling machines in use from the 1900s to the 1990s.

Unlisted - a stamp or other philatelic item not recognized by a catalog publisher.

Unofficial First Day Cover - a stamp on cover  cancelled on the first day of issue, but from a city other than the one(s) in which the stamp was officially released.

*Unwatermarked - stamp or postal stationery with no watermark.

UPU - see Universal Postal Union.

UR - Upper right.  Refers to the upper right pane from a traditional sheet of four panes.  These initials are also used to describe plate blocks taken from the upper right corner of such sheets.

USIR - a watermark found on some revenue stamps and, by error, on a printing of the $1 Woodrow Wilson stamp in the Presidential Issue (Scott 832b).  USIR is an acronym for United States Internal Revenue.

USPOD - see United States Post Office Department.

USPS - 1) the watermark used on almost all watermarked postage stamps.  Some authorities believe these initials stood for United States Postage Stamp.  See Single Line Watermark and Double Line Watermark.  2) see United States Postal Service.

USSS - see United States Stamp Society.