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Royal Mail Ship (sometimes Steam-ship, Steamer), usually seen in its abbreviated form RMS, is the ship prefix used for seagoing vessels that carry mail under contract by Royal Mail. They have the right to fly the pennant of the Royal Mail when sailing.
The designation has been used since 1840. It was used by a many shipping lines, but is often associated in particular with the Cunard line and Royal Mail Lines, which held a number of high-profile mail contract business, and which traditionally prefixed the titles of all their ships with the initials "RMS". The Canadian Pacific Railway's trans-Pacific Royal Mail contract required the construction of the first three of a fleet of steamships—the RMS Empress of China, the RMS Empress of India, and the RMS Empress of Britain which regularly sailed between Vancouver and Asia beginning in 1891.
While some lines, particularly the Royal Mail Lines, called all their ships RMS, technically a ship would use the prefix only while contracted to carry mail, and would revert at other times to a standard designation such as "SS".