The Sicilian Defence is an opening very rich in independent variations to the extent that it is always mentioned along with the mainline variation that follows. Black's purpose of playing 1...c5 is to undermine white's center by taking control of the d4 square and also to create an unbalanced position that will give him more chances of winning. It is for these to reasons why 1...c5 is the most popular response to 1.e4 ever since Chess World Champions Boris Spassky and Bobby Fischer extensively used it in the 60's and 70's. Generally, most Sicilian Defence opening variations are of a tactical rather than positional nature. Notable examples are the Najdorf Variation and the Dragon Variation, which are considered the sharpest of all chess openings.