Windsor is a historic market town and unparished area in the Royal Borough of Windsor and Maidenhead in Berkshire, England. It is the site of Windsor Castle, one of the official residences of the British monarch. The town is situated 21.8 miles (35.1 km) west of Charing Cross, central London, 5.8 miles (9.3 km) southeast of Maidenhead, and 15.8 miles (25.4 km) east of the county town of Reading. It is immediately south of the River Thames, which forms its boundary with its smaller, ancient twin town of Eton. The village of Old Windsor, just over 2 miles (3 km) to the south, predates what is now called Windsor by around 300 years; in the past Windsor was formally referred to as New Windsor to distinguish the two.