Qian Hongzun (錢弘僔) (925－June 7, 940), formally Heir Apparent Xiaoxian (孝獻世子, "the filial and wise heir apparent"), was an heir apparent to the throne of the Chinese Five Dynasties and Ten Kingdoms Period state Wuyue during most of the reign of his father Qian Yuanguan (né Qian Chuanguan, King Wenmu), but did not inherit the throne on account of his predeceasing his father.
Edward II (25 April 1284 – 21 September 1327), also called Edward of Caernarfon, was King of England from 1307 until he was deposed in January 1327.
 The fourth son of Edward I, Edward became the heir to the throne following the death of his older brother Alphonso.
 Beginning in 1300, Edward accompanied his father on campaigns to pacify Scotland, and in 1306 he was knighted in a grand ceremony at Westminster Abbey.
 Edward succeeded to the throne in 1307, following his father's death.
 In 1308, he married Isabella of France, the daughter of the powerful King Philip IV, as part of a long-running effort to resolve the tensions between the English and French crowns.
Louis of France (1 November 1661 – 14 April 1711) was the eldest son and heir of Louis XIV, King of France, and his spouse, Maria Theresa of Spain.
 As the heir apparent to the French throne, he was styled Dauphin.
 He became known as Le Grand Dauphin after the birth of his own son, "Le Petit Dauphin".
 As he died before his father, he never became king.
Duke of Rothesay (Scottish Gaelic: "Diùc Baile Bhòid" , Scots: "Duik o Rothesay" ) is a dynastic title of the heir apparent to the British throne, currently Prince Charles.
 It was a title of the heir apparent to the throne of the Kingdom of Scotland before 1707, of the Kingdom of Great Britain from 1707 to 1801, and now of the United Kingdom of Great Britain and Northern Ireland.
 It is the title mandated for use by the heir apparent when in Scotland, in preference to the titles "Duke of Cornwall" (which also belongs to the eldest living son of the monarch, when and only when he is also heir apparent, by right) and "Prince of Wales" (traditionally granted to the heir apparent), which are used in the rest of the United Kingdom and overseas.
 The Duke of Rothesay also holds other Scottish titles, including those of Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.
 The title is named after Rothesay on the Isle of Bute, Argyll and Bute, but is not associated with any legal entity or landed property, unlike the Duchy of Cornwall.
Infante Carlos of Spain (29 March 178810 March 1855) was an Infante of Spain and the second surviving son of King Charles IV of Spain and of his wife, Maria Luisa of Parma.
 As Carlos V, he was the first of the Carlist claimants to the throne of Spain.
 He is often referred to simply as 'Don Carlos'.
 He was a reactionary who was angry with liberalism in Spain and the assaults on the Catholic Church.
 He claimed the throne of Spain after the death of his older brother King Ferdinand VII in 1833.
 His claim was contested by liberal forces loyal to the dead king's infant daughter.
 The result was the bloody First Carlist War (1833–40).
 Don Carlos had support from Basque provinces and much of Catalonia, but it was not enough, and he lost the war and never became king.
 His heirs continued the arch-conservative cause, fought two more "Carlist" wars and were active into the mid-20th century, but never obtained the throne.
The Crown Prince of Thailand (or Siam; Thai: สยามมกุฎราชกุมาร ; rtgs: Sayammakutratchakuman ) is a title held by the heir apparent to the Thai throne.
 First created by King Chulalongkorn (Rama V) in 1886, for his son Prince Maha Vajirunhis, the king's eldest son by a royal wife Queen Savang Vadhana.
 Prior to this, the Siamese throne did not have a law or formal system regulating the royal succession.
 In 1688 King Petracha of Ayutthaya created the title of Front Palace, which by the Rattanakosin period had become the main title granted to the heir presumptive to the throne.
 However few Front Palaces have succeeded to the throne this way, with the exception of King Buddha Loetla Nabhalai (Rama II) in 1809.
 After the Front Palace crisis in 1875, the title of Front Palace was abolished and replaced with the title of Crown Prince, who became heir apparent to the throne.
Alphonso or Alfonso (24 November 1273 – 19 August 1284), also called Alphonsus and Alphonse and styled Earl of Chester, was an heir apparent to the English throne who never became king.
Sukhrangpha was the king of Ahom kingdom from 1332 CE to 1364 CE.
 He had to face the revolt led by his youngest brother Chao Pulai or Tai Sulai (there are confusion in Ahom historians whether Chao Pulai and Tai Sulai were same or different personality.
 Eventually he came in terms with Chao Pulai (or Tai Sulai) by appointing him as Charing Raja, a newly created official post to administer the region of Charing.
 Later the post of Charing Raja was usually conferred to the heir apparent to the throne.
 It can be compared with the title of Prince of Wales of England, where the heir apparent to the throne was conferred with this title.
Richard II (6 January 1367 – c. 14 February 1400), also known as Richard of Bordeaux, was King of England from 1377 until he was deposed on 30 September 1399.
 Richard, a son of Edward, the Black Prince, was born in Bordeaux during the reign of his grandfather, Edward III.
 His father was Prince of Aquitaine.
 Richard was the younger brother of Edward of Angoulême, upon whose death Richard, at three years of age, became second in line to the throne after his father.
 Upon the death of Richard's father prior to the death of Edward III, Richard, by primogeniture, became the heir apparent to the throne.
 With Edward III's death the following year, Richard succeeded to the throne at the age of ten.
Eustace IV (c. 1127–1135 – 17 August 1153), Count of Boulogne, was the eldest son of King Stephen of England and Countess Matilda I of Boulogne.
 When his father seized the English throne on Henry I's death in 1135, he became heir apparent to the English throne.

Question: Who was the heir apparent to the English throne who never became king, allowing his younger brother to succeed to the throne in 1307?
Answer: Alphonso