Why were the letters PX chosen for base military stores?

PX stands for Post Exchange and was chosen to differntiate it from BX (Base Exchange) used on Airforce bases.

"In the US Armed Forces, BX is a common name for a type of retail store operating on United States military installations worldwide. Originally akin to trading posts, they now resemble department stores or strip malls.

Known as the Base Exchange (BX) on Air Force bases, they are also referred to as a Post Exchange (PX) on Army posts, Navy Exchange (NEX) on Navy installations, Marine Corps Exchange (MCX) on Marine Corps installations and Coast Guard Exchange (CGX) on Coast Guard installations."

"The PX often appears in the Beetle Bailey comic strip from the 1950s.

In the television show King of the Hill, character Bill Dauterive works as a barber in the "Post Exchange Barber Shop" on the fictional "Fort Blanda" Army base.

Tokyo PX is occasionly mentioned in the TV version of M*A*S*H by soldiers given leave from the Korean War. Japan had served as a supply base for the UN forces during the conflict."


https://en.wikipedia.org/wiki/Base_exchange

PX (post exchange). The current system in the USA dates back to July 1895 when the war department issued General Order 46 directing army post commanders to establish an exchange at every post where possible. This general order set the standard for the concept and mission of today's exchange service.

The term PX was first used in 1919. In January 1942, the first supply ships arrived in Northern Ireland and the Army Exchange Service supported troops in all war theatres. In July 1948, the Army Exchange Service was redesignated as the Army and Air Force Exchange Service (AAFES) and it has supported US forces on a wide variety of deployments from Korea to the former Yugoslavia. The Air Force refers to its exchange stores as the Base Exchange (BX).

http://www.answers.com/topic/px-701

1919: The abbreviation 'PX' is first used to refer to the Post Exchange, or base store.

http://www.fundinguniverse.com/company-histories/Army-and-Air-Force-Exchange-Service-Company-History.html

http://dictionary.reference.com/browse/PX