Until 1918, German nobles didn't have any family names; these only came into existence by act of parliament when the German Republic was created. Instead, the noble families were recognized as being part of ‘Houses’, and the tradition persists. Among them, the House of Welf or Guelph is looking back on over 1,000 years of history.
Guelph is the Italian spelling of the German name Welf. The House of Welf took its name from Count Welf I who died before 826. Welf I had (probably) two sons, Conrad and Welf II (while the connection between Conrad and Welf I has been established by documents, the connection of Welf II with both is so far only surmised). Conrad became Count of Paris and was the ancestor of the Guelph in Burgundy; his descendants founded the Kingdom of Upper Burgundy in 888, inherited Lower Burgundy in 928 and unified it into the Kingdom of Burgundy in 933. King Rudolf III was the last Guelph on the throne of Burgundy. With his death in 1032, the kingdom passed by deed of will to the Holy Roman Emperors who would hold it in personal union for 500 years.
Welf II became Count in Linzgau and Alpgau and his descendants acquired large landholdings in Swabia, Rhaetia, and Bavaria. Duke Welf III of Carinthia was the last in the line of the older House of Welf; with his death in 1055, his lands passed to his sister Kunigund, the Margravine of Este. Her son Welf IV inherited his father’s lands in Italy and his mother’s lands in Germany; he was made Duke of Bavaria by King Henry IV in 1070. Welf IV took side with the Pope in the Investiture Controversy starting the long conflict between Guelphi and Ghibellini in Italy. He was the ancestor of the younger House of Welf.
His son Welf V married Duchess Matilda of Tuscany, Countess of Canossa; Matilda died childless and all her lands (including Florence) passed to Welf V. From 1070 to 1235, the House of Welf and the House of Hohenstaufen (the Ghibellini) were embroiled in a constant tug of war over the German crown with the Hohenstaufen the winners. Emperor Frederick II Barbarossa ended the feud in 1235 by creating the Duchy of Brunswick and Luneburg with Welf descendant Otto the Child as Duke Otto I. Since then, all male descendants of Otto’s were titled as Dukes of Brunswick and Luneburg until 1806. The duchy was split into individual principalities several times over the centuries between older and younger sons (there were up to five in existence at the same time).
The reorganization of Europe in 1814 led to a consolidation into the Kingdom of Hanover (House of Welf older line) and the Duchy of Brunswick (House of Welf younger line). The Kingdom of Hanover was held in personal union by the kings of the United Kingdom until the death of William IV. Inheritance laws in Hanover excluded female inheritance as long a younger male line of the House of Welf was in existence excluding Victoria from succeeding to the Hanoverian throne. Instead, the younger line took over uniting the Kingdom of Hanover and the Duchy of Brunswick-Luneburg.
The current head of the House of Welf is Ernst-August Prince of Hanover (whereby 'Prince of Hanover' is the family name) who is married to Princess Caroline of Monaco, Princess of Hanover, Hereditary Princess of Monaco (whereby 'Princess' is a personal title from the ruling house of Monaco as daughter of a Prince; her family name is 'Princess of Hanover'; and 'Hereditary Princess of Monaco' is a territorial title as the current heiress to the Principality of Monaco).