A duke is the highest of the five grades of the peerage. For guidance on how to address the four royal dukes (Cambridge, York, Gloucester and Kent) see Other Royals.
A duke is always so described, unlike the lower ranks of the peerage. If reference is made to only one duke he may be called ‘the Duke’ but if distinction is necessary, or on introduction, he should be referred to as ‘the Duke of …..’.
Ecclesiastical, ambassadorial and armed forces ranks precede the ducal rank. For example, Major-General the Duke of …..’.
When a duke is also a privy counsellor or has received a knighthood he may use the appropriate post-nominal letters.
The wife of a duke is always described as the Duchess, or the Duchess of ….. if distinction is required or on introduction.
In official and legal documents the style of The Most Noble ….. should still be used for both a duke and duchess.