The City of London is a geographically small city within Greater London, England. It is the historic core of London around which, along with Westminster, the modern conurbation grew. The City’s boundaries have remained almost constant since the Middle Ages, and hence it is now only a tiny part of the much larger London metropolis. It is often referred to as the City or the Square Mile, as it is almost one square mile (2.6 km²) in area. These terms are also often used as metonyms for the United Kingdom's financial services industry, which is principally based there.