What would a female version of Batman look like? A male version of Lara Croft? And what if we turned Bowser from Super Mario into a princess? The internet has been asking similar questions long before the advent of modern image generators — and eventually even formulated a separate "rule" for it.

Rule 63 is one of the most famous “Internet Rules”. In its classic formulation, it states: for every male character, there exists a female version, and for every female character, there exists a male version.

Of course, there is no real law here. It is rather a humorous observation of fan culture: if a character is well-known enough, sooner or later someone will draw their gender-swapped version.

Rule 63 belongs to a large and rather chaotic set of Rules of the Internet that developed within forums, imageboards, and communities associated with Anonymous.

There has never been a single definitive list of these rules: they have been rewritten, supplemented, and renumbered. Therefore, establishing the moment of appearance of each individual item is not always possible.

According to Know Your Meme, Rule 63 appeared in one of the extended versions of the list, likely in the summer of 2007. On August 8 of the same year, the definition of Rule 63 was added to Urban Dictionary.

At the same time, the idea itself is much older than its number. Fans have been gender-swapping characters in drawings, parodies, and alternate histories long before anyone called it "the sixty-third rule."

Thus, Rule 63 did not initiate the phenomenon — it simply gave a familiar fan practice a short and recognizable name.

The most typical Rule 63 art takes a well-known character and creates a version of them of the opposite gender, trying to keep key features recognizable.

A superhero might retain their costume, abilities, and personality but get a different appearance and slightly altered name. A video game hero might keep their characteristic weapon, hairstyle, or clothing colors. Sometimes the artist simply reinterprets the design, while other times they create an entire alternate universe with new versions of all characters.

In fan communities, the terms gender swap, genderbend, or genderbending are also used for this.

The classic formulation of Rule 63 is based on a simple division of "male character — female character," reflecting the lexicon of internet culture from the 2000s. Modern fan reinterpretations can be significantly more diverse, although the name Rule 63 has traditionally persisted.

Perhaps one of the most interesting examples close to Rule 63 is Fionna and Cake from the Adventure Time universe.

Finn and Jake the dog received alternate versions — Fionna and Cake. But that wasn't all. Princess Bubblegum became Prince Gumball, Marceline became Marshall Lee, and Ice King became Ice Queen.

The origins of these characters are particularly interesting. Adventure Time designer Natasha Allegri initially drew gender-swapped versions of the heroes just for fun. According to showrunner Adam Muto, the concept itself grew out of playing with fanfic tropes and gender reversal. The drawings eventually led Fionna and Cake to the actual animated series, and later the heroines received their own series Adventure Time: Fionna & Cake. This story was detailed by The Washington Post.

It turned into a nearly perfect Rule 63 story: images born from playing with fan tropes gradually became a full-fledged part of the official work.

An even larger example is Bowsette — the female version of Bowser from the Super Mario series.

In September 2018, artist Ayyk92 published a short comic that played with a new item from New Super Mario Bros. U Deluxe — the Super Crown. In the game, it allows Toadette to transform into a princess-like character named Peachette. The comic's author imagined what would happen if Bowser used such a crown.

Thus, the character with blonde hair, a dress, horns, spiked shell, and other characteristic features of Bowser was born. The image quickly gained the name Bowsette and turned into a standalone internet meme.

According to the timeline from Know Your Meme, the original comic was published on September 19, 2018. Within a few days, thousands of drawings, memes, and cosplays appeared, and in the Japanese segment of social media, the character was spread under the name Koopa-hime — "Princess Koopa."

Bowsette, however, remained a fan character and not an official heroine of Super Mario.

This case wonderfully demonstrates the mechanics of Rule 63: one artist only needs to propose a successful version of a character, after which the entire internet begins to collectively develop it.

There is also an interesting backstory that leads back to the 1990s.

In the fighting game Darkstalkers, the vampire Demitri Maximoff used a move called Midnight Bliss, which transformed the opponent into a female version before finishing the attack. When Demitri appeared in the crossover SNK vs. Capcom: SVC Chaos, the developers had to create such alternative designs for many male characters.

Players could see, for example, female versions of Ryu from Street Fighter or Terry Bogard from Fatal Fury. The idea was quickly picked up by fan artists, who began creating their own versions of Midnight Bliss for characters that weren't even in the game.

The publication Game Developer described Midnight Bliss back in 2009 as a phenomenon around which an entire mini-genre of fan gender-swap art had formed, and directly drew parallels with Rule 63.

So when Rule 63 received its name in 2007, the internet already had a ready-made culture to which it could be applied.

Not all characters that resemble the result of Rule 63 are created by fans.

In Marvel, for example, there is Lady Deadpool — Wanda Wilson, an alternate version of Deadpool from Earth-3010. She debuted in Deadpool: Merc with a Mouth #7 in 2010 and later became a member of the Deadpool Corps along with other multiversal variants of the hero. Marvel still describes her as one of the alternate versions of Deadpool. On the official Marvel website, you can also find other representatives of this strange "family": Kidpool, Dogpool, Headpool, and others.

Formally, Lady Deadpool is not a character created by Rule 63: she is an official alternate version of the hero within the Marvel multiverse. However, for illustrating the principle itself, she is a very clear example — a familiar character receives a female version with a similar appearance, role, and set of characteristic features.

Therefore, in fan discussions, the label Rule 63 is sometimes applied more broadly — not only to fan art but also to official gender-reimagined versions of well-known characters.

The boundaries of the term are quite blurred, but there are a few things worth distinguishing.

If a person simply cosplays a character of the opposite gender, it is often called crossplay. If in the plot a hero dresses up but remains the same character, that is also not necessarily Rule 63.

It is also not entirely correct to call Rule 63 any remake in which a male role is replaced with a female one. In classic Rule 63, it is important that there is an obvious connection between the original and the new version: it must be a recognizable character reimagined through a change of gender or sex.

Rule 63 works not only because the internet loves strange variations of familiar things. For artists, it is also a simple creative exercise.

How do you preserve the image of Batman while changing his gender? What would the costume, silhouette, and mannerisms remain? Should the name change? What would an entire universe look like if a similar transformation affected all the heroes?

From such questions, fan art, fanfic, cosplay, or alternate universes easily arise. And the most successful images sometimes begin to live separately from the original joke — as happened with Bowsette or Fionna and Cake.

That is why Rule 63 has survived most of the old "Internet Rules." Many items from that list today are only understood by those who remember the imageboard culture of the 2000s well. And Rule 63 still describes what is constantly happening in fan culture: give the internet a popular character — and someone will inevitably come up with a completely different version of them.