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Why ‘K’ Means Strikeout in Baseball

Baseball is full of little quirks that might not make sense at first glance. One of those quirks that people initially question is using the letter ‘K’ for a strikeout. Since the word ‘strikeout’ starts with the letter ‘S’, one would think that is the letter that would go down in the books as the official play, but instead, we use the letter ‘K’. So, why is a strikeout a “K”?

A strikeout is a “K” because Hendry Chadwick began using a “K” in the 1860s to indicate a batter struck out. At the time, “S” was already used to score a sacrifice so a “K” was used instead because it is the most pronounced letter in the word “struck”.

Six letter "K"s on an outfield fence to indicate a pitcher has six strikeouts in a game

As it turns out, “struck” used to be a more common term to describe a strikeout, so it made sense to indicate a strikeout with the letter “K”. Let’s dive more into the history of how the letter ‘K’ came to be known as a strikeout in baseball.

How Baseball Began Using The Letter ‘K’ for Strikeouts

Baseball umpire pointing to indicate a strike

One thing that has been around for as long as people can remember is the use of the letter “K” when looking at baseball box scores. But what does k mean in baseball?

In baseball, the letter “k” indicates a strikeout. “K” can be used to show a batter has struck out or “K” can be used to show how many strikeouts a pitcher acquired.

Additionally, a forward-facing k means a batter struck out swinging. A backwards k in baseball means a batter struck out looking.

Even though the word “strikeout” starts with an s, the letter k has been used to indicate a strikeout for quite some time. Let’s dive into the history of why does k mean strikeout.

Invention of the Box Score Was the Beginning of Using Letters for Plays

Baseball was invented by Abner Doubleday in 1839, but the box score was not invented until 1859 by Henry Chadwick. That means for roughly twenty years, baseball did not have any type of official scoring system for each play.

The reason Henry Chadwick invented the box score was to be a conduit between the game and its fans. Because there was no television and no photography, the best way for regular fans to follow what happened in the game was to get a play-by-play breakdown via the box score.

To be as precise and accurate as possible, Henry Chadwick resorted to using letters to symbolize what had happened during any particular play. This system of scoring is what helped popularize Chadwick’s articles and eventually landed him in the Hall of Fame in 1938.

“S” Was Already Being Used in the Box Score

When it came time for Henry Chadwick to mark down a strikeout, he had a dilemma – the letter “S” was already being used to describe a sacrifice.

Depending on who you ask, you’ll find that the letter “S” was used to indicate a “single” or you’ll find that the letter “S” was used to indicate a “sacrifice”.

As someone who is familiar with keeping score in a scorebook, I’ve always used “Sac” to describe a sacrifice hit or a sacrifice bunt. So my best guess is that “S” was used to denote a single in baseball. And if you look up what the MLB suggests, they suggest using “S” for a single.

Either way, the end result was that Chadwick was unable to use the letter “S” because it was already taken to describe another play. So he had to come up with something else to indicate if the play resulted in a strikeout.

“K” is the Last Letter in the word “Struck”

When Henry Chadwick was inventing the box score, the most popular term for a strikeout at the time was “struck”. As a way to indicate that a batter “struck three times”, he decided to use the last letter of the word “struck”.

The last letter of “struck” is also the most prominent letter in the word, so the letter “K” stuck out as the obvious choice.

In short, that is how baseball began using the letter “K” to mean strikeout! The popularity of the letter “K” caught on and we’ve been using it to describe a strikeout ever since Henry Chadwick invented it in the 1860s.

Sometimes There is a Backwards K in Baseball

Five backwards "K"s on an outfield fence to indicate a pitcher has five strikeouts in a game where the batter was looking

With the evolution of baseball, statistics also grew to include more and more ways to track the performance of players. One of the stats that naturally evolved was the use of the backwards “K” (). What is a backwards K in baseball?

In baseball, a backwards “K” (ꓘ) indicates a batter struck out looking. Regardless of how the batter received the first two strikes, the play will be marked as a backwards “K” if their last strike was called without the batter attempting to swing.

It’s difficult to tell when exactly the backward “K” became a common theme in baseball, but the best answer I could find is when the backward “K” gained its popularity in the 1980s when New York Mets fans began a tradition of keeping track of Dwight Gooden’s strikeouts.

Dwight Gooden was known by the nickname “Dr. K” and whenever he earned a strikeout, Mets fans would hang up a letter “K” to keep track of all of his strikeouts. They started using a backwards k to show which batters struck out looking.

This is how Mets fans began the tradition (that we still see to this day) of hanging a backwards “K” on the outfield fence to keep track of the strikeouts where the batters went down looking.

Some Strikeouts Are Recorded as “K-S”

We know that a k is used to record a strikeout, but sometimes you’ll see the letters “k” and “s” together. What does ks mean in baseball?

KS means a hitter struck out swinging. The play is usually recorded as “K-S”, with the letter K indicating a strikeout and the letter S indicating a swing. When put together, ks is an acronym for “swinging strikeout”.

In my experience, most people log a swinging strikeout by simply marking down a forward-facing k. But on occasion, you’ll see someone log the play as “K-S” or “KS”.

What is the “K Rate” in Baseball?

If you’re doing some research around the letter “K” meaning a strikeout then you may also come across something called a “K Rate”, but what exactly is a K Rate?

According to the MLB, the “K Rate” in baseball refers to how often a pitcher strikes out hitters. The way to calculate the K rate is to divide the total strikeouts by the total batters faced.

Sometimes, the K Rate can also be referred to as how many strikeouts the pitcher had per 9 innings.

The K Rate can tell managers how well pitchers can get an out without the hitter putting the ball in play. The higher the K Rate is, the better the pitcher typically does.

What Are the Most Strikeouts in a Baseball Game?

With all this talk of strikeouts, the letter “K”, and the backwards “K’s”, one might begin to wonder what the most recorded strikeouts are in a baseball game.

Tom Cheney holds the record for most strikeouts in a game with 21 strikeouts recorded in a 16-inning game. Max Scherzer, Kerry Wood, and Roger Clemens all hold the record for most strikeouts in a nine-inning game with 20 strikeouts.

If the fans of these teams had also followed the tradition started by the Mets, then they would have had to hang a lot of K’s!

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Steve Nelson

I'm the owner of Baseball Training World. I live in Denver, Colorado and I enjoy playing baseball in an adult baseball team in the surrounding area. Read more about Steve Nelson.