19th-century baseball journalist Henry Chadwick's score-keeping system used the letter K to indicate a strikeout because Chadwick considered it the most noticeable letter in the word "strike," which saw use more often than the longer "strikeout." Chadwick developed his scoring system from a similar system created by another sportswriter, M. J. Kelly.
In modern scorekeeping the forward-facing K, or a K-S, indicates a swinging strikeout. A backward-facing K indicates that the batter was caught looking during the strikeout. A number beside the K denotes how many the pitcher has marked down: for example, K2 means a second strikeout, swinging. The letter "S" was not used for a strikeout because it had already been taken to represent a sacrifice.
Chadwick's naming conventions include the numbering of the players based on their positions and various abbreviations like HR, used to denote a home run. As of 2015 his scoring system is still prominent and respected.