In the early 16th century, the Spanish monarchy passed to the House of Habsburg under King Charles I (also Holy Roman Emperor as Charles V), son of Queen Joanna and King Philip I of Castile. With his mother and co-monarch Joanna confined in Tordesillas, claimed to be mad, Charles I was the sole ruler, but the legal situation remained slightly ambiguous until her death in 1555 left Charles the undoubted sole monarch, though as Holy Roman Emperor it was not his principal title. Only in the reign of his son Philip II of Spain from 1556 did "King of Spain" become the usual way to refer to the monarch, in Spain and the rest of Europe.