Rule 63 is a popular internet meme originating from online fan communities, particularly on platforms like Tumblr. The concept states that for any given character, there exists an alternate version of that character that is of the opposite gender. Essentially, it suggests that if a character is male, there is a female version of that character, and vice versa. This rule often leads to the creation of gender-swapped fan art and reimagined stories featuring these alternate versions of characters.
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