Lore: twins (lore)

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Two characters, or more who are twin siblings, which come in three varieties.

Fraternal twins, the most common in reality, are the result of two egg cells being fertilized by two sperm cells and successfully implanting in the womb. Because both siblings share only 50% of their DNA, these siblings can be wildly different, having differing secondary sexual characteristics or other physical traits, and are no different than any other pair of siblings, except that their births were minutes apart instead of years.

Half-identical twins, the rarest in reality, are the result of one egg cell being fertilized by two sperm cells before successfully implanting in the womb, before separating completely into two separate embryos during cell division. Because both siblings share 75% of their DNA, these siblings will look very similar, but can have differing secondary sexual characteristics or other physical traits.
To follow TWYS, if otherwise "identical twins" have a notable difference in physical traits, such as differences in fur color, eye color, hair color, or secondary sexual characteristics, this tag should be used in place of identical twins. Similarly, if "fraternal twins" only difference is their secondary sexual characteristic, this tag should be used instead.

Identical twins, which are rare in reality, and the more commonly-tagged variety, are the result of one egg cell being fertilized by one sperm cell and successfully implanting in the womb before separating completely into two separate embryos during cell division. Because both siblings share 100% of their DNA, the resulting siblings are essentially two copies of the same person. They are always the same gender, and look extremely similar visually. Differences between them form much later in development, such as minor differences between their fingerprints.

For tagging purposes, twins usually refers to the latter.