There IS actually quite a distinct difference between the two but it has nothing to do with the Royal status of the Duke in question.
For example, Prince William is a Duke and a Royal, and yet Cambridge isn't a Duchy.
- A duchy is the territory or geopolitical entity ruled by a duke. The term implies a territorial domain, within which the duke has actual subjects and/or significant land holdings, both of which are ruled by the duke, either directly or as a vassal to a higher (Royal) authority.
- A dukedom is the title of duke, a rank of nobility, and is not necessarily attached to a duchy.
1. Duchy is a subordinate districts of a kingdom. Historic Duchies were had a distinct geographic area; the Duke was the ruler of that land, although the was subordinate to the ruler of the whole land - the Monarch.
Currently, there are only 2 Duchies in the United Kingdom - the Duchy of Cornwall and the Duchy of Lancaster. Both are "Crown bodies', regulated by Acts of Parliament. In body structure, they resemble a trust or a corporation with lands as their primary investments.
- The Duchy of Lancaster owns about 46,500 acres of land, including the Lancaster Castle. It is administered by The Chancellor of the Duchy of Lancaster, who is usually a member of the British Cabinet. The income generated by the Duchy goes directly to the Reigning Monarch of the time (the Reigning Monarch has always been The Duke of Lancaster since 1413).
It is interesting to note that only masculine form of the title exists, so even female Monarchs (such as our present Queen) are "Duke" and not "Duchess" of Lancaster.
- The Duchy of Cornwall owns about 141,000 acres of land - just over 2% of the county of Cornwall. The Estate of the Duchy has lands elsewhere as well, including in Devon and Dartmoor. The income from the Duchy is meant to support the eldest son and Heir Apparent to the Throne, and goes directly to him.
The current Duke of Cornwall is Prince Charles. The title 'Duke of Cornwall' is the second most important of his titles (after The Prince of Wales) and one his current wife, Camilla, The Duchess of Cornwall, prefers to be known under.
2. Dukedom is the title of duke, a rank of nobility; it is not necessarily attached to a duchy (a land)
There are 2 types of Dukes - Royal Dukes and Noble Dukes.
Currently, there are only 7 Royal Dukes:
- Duke of Lancaster (currently, Queen Elizabeth II)
- Duke of Cornwall and Duke of Rothesay (currently, Prince Charles)
- Duke of Edinburgh (currently, Prince Philip)
- Duke of Cambridge (currently, Prince William)
- Duke of York (currently, Prince Andrew)
- Duke of Gloucester (currently, Prince Richard - the son of Prince Henry, Duke of Gloucester, the third son of George V)
- Duke of Kent (currently, Prince Edward - the son of Prince George, Duke of Kent, the fourth son of George V)
Non-Royal (Noble) Dukes are:
- The Duke of Norfolk (currently, Edward Fitzalan-Howard, 18th Duke of Norfolk)
- The Duke of Somerset (currently, John Seymour, 19th Duke of Somerset)
- The Duke of Richmond, Lennox, and Gordon (currently, Charles Gordon-Lennox, 10th Duke of Richmond)
- The Duke of Grafton (currently, Henry FitzRoy, 12th Duke of Grafton)
- The Duke of Beaufort (currently, David Somerset, 11th Duke of Beaufort)
- The Duke of St Albans (currently, Murray Beauclerk, 14th Duke of St Albans)
- The Duke of Bedford (currently, Andrew Russell, 15th Duke of Bedford)
- The Duke of Devonshire (currently, Peregrine Cavendish, 12th Duke of Devonshire)
- The Duke of Marlborough (currently, John Spencer-Churchill, 11th Duke of Marlborough)
- The Duke of Rutland (currently, David Manners, 11th Duke of Rutland)
- The Duke of Hamilton and Brandon (currently, Alexander Douglas-Hamilton, 16th Duke of Hamilton)
- The Duke of Buccleuch and Queensberry (currently, Richard Scott, 10th Duke of Buccleuch)
- The Duke of Argyll (currently, Torquhil Campbell, 13th Duke of Argyll)
- The Duke of Atholl (currently, John Murray, 11th Duke of Atholl)
- The Duke of Montrose (currently, James Graham, 8th Duke of Montrose)
- The Duke of Roxburghe (currently, Guy Innes-Ker, 10th Duke of Roxburghe)
- The Duke of Manchester (currently, Alexander Montagu, 13th Duke of Manchester)
- The Duke of Northumberland (currently, Ralph Percy, 12th Duke of Northumberland)
- The Duke of Leinster (currently, Maurice FitzGerald, 9th Duke of Leinster)
- The Duke of Abercorn (currently, James Hamilton, 5th Duke of Abercorn)
- The Duke of Wellington (currently, Arthur Wellesley, 8th Duke of Wellington)
- The Duke of Sutherland (currently, Francis Egerton, 7th Duke of Sutherland)
- The Duke of Westminster (currently, Gerald Grosvenor, 6th Duke of Westminster)
- The Duke of Fife (currently, James Carnegie, 3rd Duke of Fife)