United States Ship (abbreviated as USS or U.S.S.) is a ship prefix used to identify a commissioned ship of the United States Navy and applies to a ship only while it is in commission. Before commissioning, the vessel may be referred to as a "pre-commissioning unit" (PCU), but is officially referred to by name with no prefix. After decommissioning, it is referred to by name with no prefix, though people commonly refer to those ships with the prefix "ex-", as in ex-ship name. In-service but non-commissioned Navy ships go by the prefix USNS, which stands for United States Naval Ship. From the early beginnings of the U.S. Navy there had been no standard method of referring to U.S. Navy ships until 1907 when President Theodore Roosevelt issued Executive Order 549 on 8 January stating that all U.S. Navy ships were to be referred to as "The name of such vessel, preceded by the words, United States Ship, or the letters U.S.S., and by no other words or letters". Today's Navy Regulations define the classification and status of naval ships and craft: