Edward III had seven sons. Of those who survived infancy the four eldest were: his heir - Edward (the Black Prince), Lionel (Duke of Clarence), John of Gaunt (Duke of Lancaster), and Edmund Langley (Duke of York). As the Black Prince predeceased his father by a year, it was his son who eventually succeeded to the throne in 1377, becoming Richard II.
Richard's reign witnessed both peasant revolts and aristocratic conspiracies. Successive disputes with the nobility eventually led to his downfall. In 1399, Richard's cousin Henry Bolingbroke, the eldest son of John of Gaunt (Edward III's third son), usurped the throne, had Richard executed and founded the ruling dynasty of Lancaster as Henry IV.
Henry's claim to the throne rested on his being the next surviving male heir. Unfortunately for him, there was a rival claim through the line of Philippa, daughter of Edward III's second son - the Duke of Clarence. Philippa had married Edmund Mortimer, Earl of March, by whom she had a son; thus founding the claim of the House of Mortimer.
In 1425, Richard Duke of York, the grandson of Edmund Langley (Edward III's fourth son) was to inherit the Mortimer claim through his maternal uncle. He was to prove a formidable rival to his distant cousin, the weak-minded Henry VI of the House of Lancaster who alienated the Duke by borrowing vast sums of money he never repaid.