In philately

Philately

Philately is the study of stamps and postal history and other related items. Philately involves more than just stamp collecting, which does not necessarily involve the study of stamps. It is possible to be a philatelist without owning any stamps...

, a ship cover is a cover

Cover (philately)

In philately, the term cover pertains to the outside of an envelope or package with an address, typically with postage stamps that have been cancelled and is a term generally used among stamp and postal history collectors. The term does not include the contents of the letter or package, although...

 that was mailed aboard a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

, while a naval cover is one posted on a navy

Navy

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

 vessel. Shipboard postal facilities are ubiquitous on ships of any size, although the "post office

Post office

A post office is a facility forming part of a postal system for the posting, receipt, sorting, handling, transmission or delivery of mail.Post offices offer mail-related services such as post office boxes, postage and packaging supplies...

" may consist only of an alcove and a sailor working it part-time.

Ship covers usually carry a postmark

Postmark

thumb|USS TexasA postmark is a postal marking made on a letter, package, postcard or the like indicating the date and time that the item was delivered into the care of the postal service...

 unique to the vessel. Collectors of ship covers will look for different types used at different periods, as well as for covers indicating routing through particular ports, and so forth.

The Universal Ship Cancellation Society

Universal Ship Cancellation Society

Universal Ship Cancellation Society is an international philatelic non-profit organization with an interest in postmarks and covers from all maritime services...

, founded in 1932, specializes in the study of naval covers, particularly those of the US Navy.