A Spanish priest who was the strongest chess player in Europe for a short while in the 16th century, Ruy Lopez wrote the first known book to concentrate mainly on chess openings (Livro de la Invencion liberal y Arte del juego del Axedrez, or, Book of the liberal invention and art of the game of chess), in which he introduced the opening which still bears his name and remains perhaps the most popular opening in the history of chess - The Ruy Lopez, or Spanish Opening. He also coined the term 'gambit' to describe the sacrifice of material in the opening in return for the gain of time, and he suggested the rule, still in use in modern play, that after 50 moves in which no piece is captured and no pawn is moved, the game is automatically drawn.
Chess openings at that time were not played according to any particular system, and players tended to just move their pieces around and hope that everything would work out all right when the time actually came to calculate. Apparently we owe the writing of his book (the first truly analytical study of the opening in chess) to the interest of Philip II of Spain, who, when asked what he wanted for Christmas in 1561, said that he would like a book about chess.
Ruy Lopez, who was his favourite priest, went to Italy, the chess-playing capital of the modern world, to look for such a book, and while he was there defeated the two leading players of the time. He bought a book called Questo libro e da imparare giocare a scachi et de li partiti which was published in 1512, but he didn't think it was any good, so on his way home to Spain he wrote his own book. Besides its analysis and theory, it also contained a lot of advice to the unscrupulous player, such as "Always make sure your opponent is seated with the sun in his eyes," and "Make sure your opponent has eaten heavily before the game".
Apparently Italian national honour was so dented by this escapade that, after 12 years of training and study, the two best Italian players (Leonardo da Cutri and Paolo Boi) were sent to Spain to play Ruy Lopez. This time they both won.