Greater London is often referred to as "the City" or "the Square Mile" and it forms the main financial district. Archaically, the urbanised area of London was known as the Metropolis. In common usage, the terms "London" and "Greater London" are usually used interchangeably. Greater London is officially divided for some purposes, with varying definitions, into Inner London and Outer London. For some strategic planning purposes it is divided into five sub-regions. Greater London is under the strategic local governance of the Greater London Authority (GLA). It consists of an elected assembly, the London Assembly, and an executive head, the Mayor of
City of London up the centre of the Thames, with the exception that Blackfriars Bridge falls within the City; the City controls London Bridge (as part of Bridge ward) but only half of the river underneath it, a feature which is unique in British local administration. The boundaries are marked by black bollards bearing the City's emblem, and by dragon boundary marks at major entrances, such as Holborn. A more substantial monument marks the boundary at Temple Bar on Fleet Street. In some places the financial district extends slightly beyond the boundaries, notably to the north and east, into the London Boroughs of