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Scotland Yard is a wonder in vanilla Civilization IV and Warlords, but a building in Beyond the Sword.
Strategy
Vanilla & Warlords
Scotland Yard is required to train Spies, and adds 1 Great Scientist point per turn.
Beyond the Sword
Scotland Yard can be built by a Great Spy, and provides a +100% bonus to 
Espionage output.
Civilopedia Entry
Scotland Yard is the name of the British police force having jurisdiction over the Greater London area. Scotland Yard is named for the neighborhood in which the original police headquarters stood, and the force retains that nickname even though the headquarters have since been relocated (to "New Scotland Yard").
Scotland Yard was established in 1829 by act of Parliament. According to one of the new force's commissioners, "The primary object of an efficient police is the prevention of crime; the next that of detection and punishment of offenders if crime is committed. To these ends all the efforts of police must be directed. The protection of life and property, the preservation of public tranquility, and the absence of crime, will alone prove whether those efforts have been successful and whether the objects for which the police were appointed have been attained."
In the early to mid-19th century Scotland Yard was the most advanced police force in the world. It gained worldwide notoriety from its pursuit of Jack the Ripper, and it was immortalized in the Sherlock Holmes stories of Sir Arthur Conan Doyle. To this day the modern Scotland Yard remains one of the finest police forces to be found anywhere.