Kate married William in 2011, and that's when she officially became Catherine Elizabeth, Her Royal Highness the Duchess of Cambridge, as listed on Prince George's birth certificate. Just like William, the palace also refers to Kate as Catherine. During an interview with BBC's Radio 1 in April, Scott Mills questioned the fact that Prince Harry referred to his sister-in-law as "Cath" in a recent video, asking, "Everyone calls you Kate, don't they?" "I will answer to most things," she replied. Well, there you have it.