And for my last post of the day with a 'Royal Wedding' theme, I'll look at the word 'troth' which featured prominently in the ceremony, along with other archaisms.
Troth developed from the Old English word for 'truth'. It originally meant faithfulness, loyalty or honesty. The word on its own died out in the 16th century, only remaining in certain set phrases, such as plight one's troth. To plight one's troth means 'to pledge one's faith' or 'to make a solemn promise', particularly a promise to marry. The slightly more common, although old-fashioned or literary, word 'betrothed' is from the same root.