Duke

The term duke is a title of nobility which refers to the sovereign male ruler of a Continental European duchy, to a nobleman of the highest grade of the British peerage, or to the highest rank of nobility in various other European countries, including Portugal, Spain and France (in Italy, principe is held to be the highest grade). The wife of a duke, or a woman who rules a duchy, is known as a duchess.

Elsewhere

• In Denmark, the main duchy being Schleswig, part of which later was transferred to Germany; its southern neighbour Holstein, also under the Danish crown (both in personal union), was always a German principality
• in splintered Poland, notably Masovia