In actuality, there are three STALKER games — Shadow of Chernobyl, Clear Sky, and Call of Pripyat ... like the implementation of a radial menu for easier gear and inventory management ...
STALKER: Shadow of Chernobyl was the groundbreaking first foray into the Zone, STALKER: Clear Sky was a prequel, and STALKER: Call of Pripyat was a sequel set just weeks after the first game.