"Elementary, my dear Watson."

When the ____________ were being established in 1829, one of the first tasks was to find a building to act as the new headquarters. The building was at 4 Whitehall Place. The new building was adjacent to the Public Carriage Office where a Commissioner was already responsible for the licensing of taxi cabs. When the public went to see the new Commissioners, they used the back entrance of 4 Whitehall Place the rear of which was converted into a police station. The reception area soon became known as "Back Hall", an expression still used today in the headquarters of the ___________. This entrance was in Great X, off Whitehall, and the building soon became known as X.

Id X.