Botvinnik used almost exclusively pawn openings with the white pieces. In his eight World Championship matches, he never started a game with an e4-opening, and his usual choices as White were the English Opening or Queen's Gambit. When playing the black pieces, he preferred the French Defense or Sicilian Defense in response to 1.e4, and the Slav Defense or Nimzo-Indian Defense in response to 1.d4. While Botvinnik did not use a wide range of openings, he made major contributions to those he did use, for example: the Botvinnik Variation of the Semi-Slav Defense in the Queen's Gambit Declined, the Kasparov/Botvinnik system in the Exchange Variation of the Queen's Gambit Declined, the Caro–Kann Defence (both the Panov–Botvinnik Attack for White and various approaches for Black), the Winawer Variation of the French Defense, the Botvinnik System in the English Opening. In his openings research Botvinnik did not aim to produce tactical tricks that would only be effective once, but rather systems in which he aimed to understand typical positions and their possibilities better than his rivals. His advice to his pupils included "My theory of the openings fitted into one notebook" and "You don’t have to know that which everyone knows, but it is important to know that which not everyone knows." In fact he used different notebooks in different periods, and copied a few analyses from one notebook to the next.