The Key is a smock mill in Sleat, Friesland, Netherlands which was built in 1755 and is in working order. The mill is listed as a Rijksmonument.
The mill stands on the Lemster Waterpoort. A post mill stood here in the sixteenth century. it was destroyed in 1593. Another post mill replaced it. It was shown on a map dated 1664 and in a drawing dated c. 1723.
De Kaai was built in 1755. For many years it was known as De Korenmolen (English: The Corn Mill). It was given its name in 2006, derived from its position by one of the two water gates in Sleat. An early photograph shows that the mill originally had a wooden windshaft and the smock tower had vertical boards. In 1829, the mill was owned by Gerhard Wenzel ten Brink, a merchant in Sleat. On 9 June 1870, it was sold to Daniël Ruurd Westra, passing to his son Fedde in 1890. On 7 October 1897. the mill was bought at auction by Thijs Bosma for ƒ925. He later sold the mill privately to Pieter Gerrits Bosma for ƒ900. By the 1920s, the mill was derelict