If not for the evidence supporting Chadwick's earlier use of "K", this explanation would be reasonable. Those unaware of Chadwick's contributions have speculated that "K" was derived from the last name of 19th century pitcher Matt Kilroy. Chadwick was responsible for several other scorekeeping conventions, including the use of numbers to designate player positions. Chadwick decided to use "K", the last letter in "struck", since the letter "S" was used for "sacrifice".
As is true in much of baseball, both the box score and scorecard remain largely unchanged to this day. The use of "K" for a strikeout was invented by Henry Chadwick, a newspaper journalist who is widely credited as the originator of the box score and the baseball scorecard. Virtually every televised display of a high-strikeout major league game will include a shot of a fan's strikeout display, and if the pitcher continues to strike out batters, the display may be shown following every strikeout. The "K" may be placed backwards ( ꓘ) in cases where the batter strikes out looking, just as it would appear on a scorecard. One baseball ritual involves fans attaching a succession of small "K" signs to the nearest railing, one added for every strikeout notched by the home team's pitcher, following a tradition started by New York Mets fans in honor of "Dr. "K" is still commonly used by fans and enthusiasts for purposes other than official record-keeping. Despite the scorekeeping custom of using "K" for strikeout, "SO" is the official abbreviation used by Major League Baseball. In terms of game play, swinging and looking strikeouts are exactly equivalent the difference in notation is simply to record this aspect of the time at bat.
A strikeout looking (where the batter does not swing at a pitch that the umpire then calls strike three) is often scored with a backwards K ( ꓘ), and sometimes as a K-L, CK, or Kc (the 'c' for 'called' strike). In baseball scorekeeping, a swinging strikeout is recorded as a K, or a K-S. Should the catcher field the ball and step on home plate before the runner from third base can score, then the runner from third base is forced out. The batter-runner is obliged to run for first base and other base-runners are obliged to attempt to advance one base. With the bases loaded and two strikes with two outs, the catcher drops the ball or catches it on the bounce. It is also possible for a strikeout to result in a fielder's choice. Because of this, a pitcher may occasionally be able to record more than three strikeouts in one inning.
When this happens, a strikeout is recorded for both the pitcher and the batter, but no out is recorded. In Major League Baseball, it is known as an uncaught third strike. In Japan, this is called furinige ( 振り逃げ), or "swing and escape". Thus, it is possible for a batter to strike out, but still become a runner and reach base safely if the catcher is unable to catch the third strike cleanly, and he then does not either tag out the batter or force him out at first base.