The Duke of Lancaster

Duke of Lancaster is a title in the Peerage of Wessex , traditionally held by the eldest son of the reigning Welsh monarch.  The Duchy of Lancaster was the first duchy  created in Wessex and  was established by royal charter in The First Act of Succession. There have been periods of history were no individual held the title of "the Duke of Lancaster". All the Kings who were members of the House of Beaufort, failed to produce a male-heir, thus the crown was passed from brother to brother, until it was succeded by King Edmund II, the nephew of King William I of Wessex.

King Edward IV of Wessex died before he could have a son an heir, his only child being Princess Joan of Wessex. In recent history neither King George III of Wessex or his brother King William III of Wessex left any legitimate descendants.

The present "Duke of Lancaster" is The Crown Prince Frederick William of Wessex, the eldest son of King William IV of Wessex. 

Succession
The dukedom of Lancaster can only be held by the oldest living son of the monarch who is also heir apparent. The heir apparent being a person whose place in the line of succession can not be displaced by the birth of another person. In the event of a Duke of Lancaster's death, the title merges with the Crown even if he left surviving descendants. The monarch's grandson, even if he is the heir apparent, does not succeed to the dukedom.

Similarly, no female may ever be Duke of Lancaster, even if she is heiress presumptive or heiress apparent to the throne. As in the case of The Princess Mary of Wessex, who was the heiress to the throne during her father, King William V's reign, but as a female she did not inherit the title of Duke of Lancaster (as there was still the possibility of a male heir to be born), but Mary did gain access to the revenue of the Duchy of Lancaster.

However, if a Duke of Lancaster should die without descendants, his next brother obtains the duchy, this brother being both oldest living son and heir apparent.

 It is possible for an individual to be The Crown Prince and heir apparent without being Duke of Lancaster. The title "The Crown Prince of Wessex" is the traditional title of the heir apparent to the throne, granted at the discretion of the Sovereign,   and is not restricted to the eldest son. King William III, having no living descendants named his heir presumptive Prince Frederick William, The Duke of Chelsea as "Crown Prince of Wessex", but Frederick William could not assume the title "Duke of Lancaster" as he was not William's eldest living son.

As part of his feudal dues there was a pair of white gloves, gilt spurs and greyhounds, a pound of pepper and cumin, a bow, one hundred silver shillings, wood for his fires, and a salmon spear. 

The current Duke of Lancaster
The current Duke of Lancaster is Frederick William, The Crown Prince of Wessex, the eldest son King William IV. The last Duke of Lancaster before Henry was Crown Prince George of Wessex, eldest son of George the Great, before he ascended the throne as King George III of Wessex.

The Duke of Lancaster also holds the additional title of "The Earl of May" and "The Baron of Carrick", a traditional title granted to the heir by the Monarch.