The Plantagenets were descended from the Norman line. William the Conqueror's fourth son, Henry I, had his only legitimate son predecease him by some 15 years in 1120. Originally his daughter Matilda was married to Henry V of the Holy Roman Empire, but he struggled to maintain power, dying young in 1125.

Matilda became the sole heir of Henry I and, to help cement the English position in France as not only Kings and Queens of England but also as the Dukes and Duchesses of Normandy, he married her off to Geoffrey Plantagenet, count of Anjou.

When Henry I died in 1135 a civil war erupted in England whereby Matilda's cousin, Stephen of Blois, seized the English throne. Matilda and her husband Geoffrey rallied their allies and battled Stephen's armies for 18 years until both sides were almost totally exhausted of resources and money.

Peace was achieved through the Treaty of Wallingford, whereby Matilda renounced her claim on the English throne in exchange for Stephen naming her son, Henry II, his heir.

The Plantagenets are descended from Matilda, the granddaughter of William the Conqueror, but take their name from her husband, Geoffrey Plantagenet, the count of Anjou.

A similar situation is present in the current British royal family. The descendants of the current Queen (Elizabeth Windsor) and her husband (Philip Mountbatten) often use the last name 'Mountbatten-Windsor' on legal documents. As an interesting side note, both Mountbatten and Windsor are made up names from 1917 when the royal family changed them to look less German. Had they not done so their last name would today be 'Battenberg-Saxe-Coburg and Gotha'.