Question: The Sovereign also holds the title Duke of Lancaster, regardless of gender. Who does this title currently describe?

Context articles: Duke of Lennox
Duke of Buccleuch
Alan Reid (courtier)
Abdication of Wilhelm II
Prince Aimone, Duke of Aosta
Federal prince
Duke of the Franks
Duke of Aosta
Duke of Lancaster
Duke of Abercorn

The title Duke of Lennox has been created several times in the Peerage of Scotland, for Clan Stewart of Darnley.
 The Dukedom, named for the district of Lennox in Stirling, was first created in 1581, and had formerly been the Earldom of Lennox.
 The second Duke was made Duke of Richmond; at his death, the dukedom of Richmond became extinct.
 The fourth Duke of Lennox was also created Duke of Richmond; at the death of the sixth Duke, both the dukedoms became extinct.
 The Dukedom of Richmond and one month later that of Lennox were created in 1675 for Charles Lennox, an illegitimate son of Charles II.
 The Duke of Richmond and Lennox was created Duke of Gordon in 1876.
 Thus, the Duke holds four dukedoms (including Aubigny-sur-Nère), more than any other person in the realm.
The title Duke of Buccleuch , formerly also spelt Duke of Buccleugh, is a title created twice in the Peerage of Scotland.
 The second creation dates to the 20 April 1663.
 The Dukedom was for the Duke of Monmouth, who was the eldest illegitimate son of Charles II of Scotland, England, and Ireland, and who had married Anne Scott, 4th Countess of Buccleuch.
 The Duke also holds the Dukedom of Queensberry, created in 1684.
Sir Alan Reid, GCVO has been Keeper of the Privy Purse, Treasurer to the Queen and Receiver General to the Duchy of Lancaster in the Royal Household of the Sovereign of the United Kingdom since 2002.
 As Keeper of the Privy Purse, Reid is responsible for the expenditure of public funds voted by the Parliament to the Sovereign, usually called the Sovereign Grant.
 As Treasurer to the Queen, he is also responsible for the Sovereign's private finances.
 The Sovereign also holds the title of Duke of Lancaster (regardless of gender), which brings with it responsibility for the Duchy of Lancaster estates.
Kaiser Wilhelm II abdicated as German Emperor and King of Prussia on 28 November 1918 from exile in Amerongen.
 He had ruled Germany and Prussia from 15 June 1888 until 9 November 1918, when he went into exile.
 Following the abdication statement and German Revolution of 1918–19, the German nobility as a legally defined class was abolished.
 On promulgation of the Weimar Constitution on 11 August 1919, all Germans were declared equal before the law.
 Altogether abolished were titles borne exclusively by monarchs; e.g., emperor/empress, king/queen, grand duke/grand duchess, etc.
 There were 22 federal princes of the Kaiserreich ("within Germany"), who lost their titles and domains.
 Of these princely heads of state, 4 held the title King (König) (the Kings of Prussia, Bavaria, Saxony, and Württemberg), 6 held the title Grand Duke (Großherzog), 5 held the title Duke (Herzog), and 7 held the title Prince (i.e. Sovereign Prince, "Fürst").
Prince Aimone, 4th Duke of Aosta ("Aimone Roberto Margherita Maria Giuseppe Torino"; 9 March 1900 – 29 January 1948) was a prince of Italy's reigning House of Savoy and an officer of the Royal Italian Navy.
 The second son of Prince Emanuele Filiberto, Duke of Aosta he was granted the title Duke of Spoleto on 22 September 1904.
 He inherited the title Duke of Aosta on 3 March 1942 following the death of his brother Prince Amedeo, in a British prisoner of war camp in Nairobi.
Federal prince (German: "Bundesfürsten" , "Princes of the Federation") was the generic term for the royal heads of state (monarchs) of the various states making up the German Empire, with the exception of the states that were republics and Alsace-Lorraine which had a special status.
 The empire was a federal state, with its constituent states remaining sovereign states.
 Among the states, the majority were monarchies, 3 states were republics and Alsace-Lorraine was an imperial territory ruled by a steward.
 The states became part of the German Empire by an 1871 treaty.
 The head of the empire was granted the title German Emperor (with the title "Emperor of Germany" being deliberately avoided), and was simultaneously a federal prince as King of Prussia, the sovereign of its largest federal state.
 Of the princely heads of state, 4 held the title King (König) (the Kings of Prussia, Bavaria, Saxony, and Württemberg), 6 held the title Grand Duke (Großherzog), 5 held the title Duke (Herzog), and 7 held the title Prince (i.e. Sovereign Prince, "Fürst").
 The heads of state of the city republics, where the concepts of nobility and royalty did not exist, were of equal standing to the federal princes and held the titles First Mayor and President of the Senate (Hamburg), Mayor and President of the Senate (Lübeck), and President of the Senate and Mayor (Bremen).
 The heads of state of the city republics were regarded as primi inter pares among the senators according to the states' republican constitutions, and these states were ruled by an oligarchy of hereditary Grand Burghers, also known as Hanseaten or patricians.
The title Duke [and Prince] of the Franks ("dux" ["et princeps"] "Francorum") has been used for three different offices, always with "duke" implying military command and "prince", on those occasions when it was used either with or in preference to "duke", implying something approaching sovereign or regalian rights.
 The term "Franks" may refer to an ethnic group or the inhabitants of a territory called "Francia", named after the original Franks.
 The first office was that of the mayors of the palace of the Merovingian kings of the Franks, whose powers increased as those of the kings declined.
 The second was that of the second-in-command to the early kings of France, the last incumbent of which succeeded to the throne in 987.
 This title was sometimes rendered Duke of France ("dux Franciae").
 The third instance was that of the rulers of the Frankish-inhabited lands of Germany, the so-called "tribal" duchy of Franconia.
In the mid-13th century the Hohenstaufen Holy Roman Emperor Frederick II made the County of Aosta a duchy; its arms were carried in the Savoyard coat of arms until the unification of Italy in 1870.
 The region remained part of Savoy lands, with the exception of a French occupation, 1539—1563.
 The title Duke of Aosta was given to various princes of the dynasty of Sardinia, second sons of the reigning monarch.
 It can be compared to the English Duke of York, French Duke of Orléans, Swedish Duke of Södermanland and the Scottish Duke of Albany.
 It remained in the branch of Prince Amedeo of Savoia, the second son of king Victor Emanuel II of Italy, as he was the first ever cadet prince Duke of Aosta who left male heirs.
 The subsidiary titles of the Duke of Aosta are, from the heritage of Maria Vittoria dal Pozzo, the mother of Duke Emanuele Filiberto, Prince della Cisterna and of Belriguardo, Marquess of Voghera, and Count of Ponderano.
 Ponderano was created in 1559, Voghera in 1618; Cisterna and Belriguardo as princely in 1670.
 The title has been used since July 2006 by Amedeo's son Prince Aimone, Duke of Apulia (born 1967), who is married to Princess Olga of Greece, younger daughter of Prince Michael of Greece and Denmark.
The Duke of Lancaster is an ancient title which is informally used within Lancaster to describe Elizabeth II, the monarch of the United Kingdom and owner of the estates of the Duchy of Lancaster.
 The Duchy of Lancaster exists as a separate entity from the Crown Estate and currently provides income for the British monarch.
The title Duke of Abercorn is a title in the Peerage of Ireland.
 It was created in 1868 and bestowed upon James Hamilton, 2nd Marquess of Abercorn.
 Although the Dukedom is in the Peerage of Ireland, it refers to Abercorn, West Lothian, and the Duke also bears four titles in Peerage of Scotland and two in the Peerage of Great Britain, and is one of only three peers who have titles in those three peerages.
 The Duke of Abercorn also bears the French title of Duke of Châtellerault, created in 1548.