In the 19th century, Marie-Antoine Carême anointed Béchamel, Velouté, Espagnole, and tomato sauce as the building blocks for all other sauces in his work L'Art de la Cuisine Française au Dix-Neuvième Siecle. Later on, Hollandaise got added to the family. Since then, many people consider others sauces -- sweet and savory from all around the world -- as unofficial extended relatives of these five sauces.
http://food52.com/blog/12209-the-five-mother-sauces-every-cook-should-know