I learnt about Robert the Bruce when I was a young schoolchild. He defeated Edward II at the Battle of Bannockburn in 1314 which secured Scottish independence. Not that that was what I learnt at school. Robert the Bruce used to feature in worthy tomes for children. He apparently watched a spider struggle across a cave's wall; every time the spider fell it started clambering up the wall again, and we were all supposed to follow this 'if at first you don't succeed, try, try again' philosophy.
Although I remembered this Scottish king and the story, it never occurred to me till many years later to wonder why he was called Robert the Bruce. After all, no-one says Gordon the Brown, or David the Beckham. I discovered the answer the other day whilst browsing the 'The' entry in the Oxford English Dictionary. 'The' here is an English representation of the French word de, meaning 'of'. Robert's ancestors were Scoto-Norman and he was called in French Robert de Brus or Robert de Bruys. Similarly Simon de Montfort is sometimes called Simon the Montfort.
I don't suppose many people look up the word 'the' in the dictionary, but I find looking up common words in dictionaries fascinating, especially in the OED, which is a historical dictionary. It's often amazing the history these words have had. 'The' is one of the oldest words in the dictionary; there are citations relating to the early 9th century. Not that they are very readable; at the time 'the' was spelled with the Old English letter thorn. 'The' used to decline grammatically and the cases are given in the dictionary: nominative, accusative, dative, genitive and instrumental. There were also three genders: masculine, feminine and neuter. 'The' was originally a demonstrative as well as definite article ie it meant 'this'.
The letter thorn which began 'the' over a thousand years ago gradually changed in shape until it eventually resembled a y. That's why some gift and touristy shops like to call themselves Ye Olde Gifte Shoppe' or something similar; 'ye' is what the Middle English version of 'the' looked like when written (it was still pronounced with a 'th' sound).