It is not an automatic title but rather is granted to the heir by the monarch. Prince Charles, for example, did not become Prince of Wales until 1958 when he also became Earl of Chester. Generally the eldest daughter of the monarch is given the title of Princess Royal. Princess Anne became the Princess Royal in 1987. Princes and Princesses generally are known by their title followed by their first name only.
Signatures usually consist of their first names only.
As in the case of Mark Phillips, a husband without a title does not gain one when he marries a princess, unless the Queen grants him his own title. Princess Margaret's former husband, for example, was named Earl Snowdon when he married her. A woman marrying a prince takes her husband's title and name. Prince Michael's wife, for example, is known as Princess Michael of Kent. In the 18th and 19th centuries the grandchildren of the monarch also were styled prince or princess.