Chess opening are so complex and theoretical that amateur players will have trouble understanding them. An example is the Perenyi Attack of the Sicilian Defense (see diagram) which yields an immensely complicated and tactical position that even strong players have difficulty handling, and that is beyond the comprehension of most amateurs. Major changes in the rules of chess in the late fifteenth century increased the speed of the game, consequently emphasizing the importance of opening study. Thus, early chess books, such as the 1497 text of Luis Ramirez de Lucena, present opening analysis, as does Pedro Damiano (1512), and Ruy López de
Ruy Lopez e5 and placing the queen on the c-file which may later become open after ...cxd4. Other Black moves in this position are 11...Bb7 and 11...Nd7; the latter was adopted by Keres a few times in the 1960s. The Chigorin Variation has declined in popularity because Black must spend some time bringing his offside knight on a5 back into the game. The Chigorin is divided into four "ECO" classifications. In C96, Black or White deviate after 10.Bc2, and do not reach the classical main line position 10...c5 11.d4 Qc7. In C97, White proceeds from the diagram with 12.a4, 12.d5, 12.b4, or