Why is a strikeout called a "K"?

In baseball scorekeeping, a swinging strikeout is recorded as a K, or a K-S. A strikeout looking (where the batter does not swing at a pitch that the umpire then calls strike three) is sometimes scored with a backwards K.

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. Both the box score and scorecard persist largely unchanged to this day, as the game itself is largely unchanged. The letter "S" was used for "sacrifice" so Chadwick decided to use "K", being the last letter in "struck". Chadwick also invented many other baseball scoring abbreviations, such as using numbers to designate player positions, progressing from the pitcher [1], catcher [2], through the infield, with the shortstop [6] counted after the basemen, to the right fielder [9].[2]

That Chadwick first established the convention of using the "K" abbreviation is well-founded, with reliable and authentic primary materials surviving (see citation above). Those unaware of Chadwick's contributions have speculated that "K" was derived from the 19th century pitcher Matt Kilroy's last name. If not for the evidence supporting Chadwick's earlier use of "K", this speculation would be reasonable: Kilroy did much to raise the prominence of the strikeout, setting an all-time record of 513 strikeouts in 1886, only two years after overhand pitching was permitted. Kilroy's record, however, is forever confined to its era: the pitcher's mound during his record-setting season was only 50 feet (15 m) from the batter; it was moved to its current distance of 60'6" in 1893. The modern record (1901–) is 383 strikeouts, held by Nolan Ryan, one better than Sandy Koufax's 382.

Although some people use "K" to record pitchers' strikeouts, "SO" is the official abbreviation used by Major League Baseball.[3]

In addition, "K" is still commonly used by fans and enthusiasts for purposes other than official record-keeping. In one baseball ritual, fans at the ballpark who are seated in view of the batter (and the television cameras) attach a succession of small "K" signs to the nearest railing, one added for every strikeout notched by the home team's pitcher. As is traditional for those who keep a record of the game on paper, the "K" is placed backwards in cases where the batter strikes out looking. Virtually every televised display of a major league game in which a pitcher registers a high number of strikeouts (7 or 8) will include a shot of a fan's strikeout display, and if the pitcher continues to strike out batters, the display often will be shown following every strikeout. In the event that a known "strikeout pitcher" is on the mound, the strikeout display will be televised from the beginning.

Because it is made with three strokes of the pencil.

Henry Chadwick and another writer named M.J. Kelly are largely responsible for the scoring system baseball uses today. Chadwick already had "S" slated for "sacrifice." So a strikeout became a "K", after the last letter of the word "struck." The reason a strikeout isn't a "T" is because "struck" was the preferred term of the day.

Another theory is that "K" is used because there are three lines in the letter and three strikes in a strikeout.