Grosvenor Square is a large garden square in the exclusive Mayfair district of London, England. It is the centrepiece of The Mayfair property of the Duke of Westminster, and takes its name from their surname, "Grosvenor". Many of the houses were rebuilt later in the 18th century or during the 19th century, generally acquiring an extra storey when this happened. Number 23 (later 26) was rebuilt in 1773–74 for the 11th Earl of Derby by Robert Adam, and is regarded as one of the architect's finest works and as a seminal example of how grandeur of effect and sophisticated planning might be achieved on a confined site. It was demolished and rebuilt again in the 1860s. Nearly all of the older houses were demolished during the 20th century and replaced with blocks of flats in a neo-Georgian style, hotels and embassies.
The Central garden, which was originally reserved for the use of the occupants of the houses as was standard in a London square, is now a public park managed by The Royal Parks. At the eastern end of the garden there is a small memorial dedicated to the British victims of the September 11 attacks. The memorial includes an elliptical granite block engraved with the names of the victims and the poem For Katrina's Sun-Dial, by Henry van Dyke.