The Côte d'Azur is the name given to the Mediterranean coastline of southeast France, extending roughly from the Franco-Italian border westwards to Toulon. It includes the Principality of Monaco which is divided into four traditional quartiers (or districts), one of which is Monte Carlo. British people often refer to the Côte d'Azur as the French Riviera.
Since the 18th century, the French Riviera has been a popular holiday destination for the British upper classes and royalty. During the first half of the 20th century, it also attracted many writers and artists, including W. Somerset Maugham and Pablo Picasso. Later on, it became the haunt of various celebrities in the world of film and popular music, including Brigitte Bardot and Elton John. Amongst the resorts of the Côte d'Azur, Monte Carlo, with its world-famous casino, has a particular reputation for attracting the wealthy and famous.
There is no record of a specific hotel named the Côte d'Azur in Monte Carlo, but Daphne du Maurier may have had a luxury hotel such as the Hotel de Paris Monte Carlo (founded 1863) in mind when she created the Côte d'Azur.