- a strait between Anglesey Island and the mainland of NW Wales. 14 miles (23 km) long.
- a channel of the Irish Sea between the island of Anglesey and the mainland of NW Wales: famous suspension bridge (1819–26) designed by Thomas Telford and tubular bridge (1846–50) by Robert Stephenson. Length: 24 km (15 miles). Width: up to 3 km (2 miles)