Half-identical twins, or Semi-identical twins, scientifically known as "Sesquizygotic Twins", are twins who share 75% of their DNA, having nearly identical physical traits accompanied by notable differences
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Half-identical twins are the result of one egg cell being fertilized by two separate sperm cells, after which it successfully implanted in the womb before undergoing cell division and separating completely into two embryos.
Both siblings share 75% of their DNA as a result. This means that the twins have more in common genetically than normal siblings or fraternal twins and less in common genetically than identical twins. This means these siblings will develop to have many visual similarities, but with at least some visual differences from one another, which can include but aren't limited to, having differing sexual characteristics or hair, eye, and fur colors.

During development, both embryos share the same placenta and amniotic sac.

Half-identical twins have the potential to be visually indistinguishable from either normal siblings or identical twins. To Tag What You See (TWYS), this should only be tagged given evidence in the image.

This tag should be used if one of the following differ in otherwise identical twins:

• Fur, Skin, or Hair Color, Patterns or Traits
• Examples: Different countershading, skin tone, or type of hair.
• Exclusions: scars, tattoos, bleached or dyed, or differing hairstyles
• Eye Colors
• Examples: Differences to sclerae, iris, and pupils.
• Even if all three are different, for the sake of tagging, it counts as one difference.
• Exclusions: Ocular Prosthetic, glass eye, or other non-genetic differences in eyes.
• Body Composition
• Examples: differences in height, greater or fewer body parts, or genetic defects.
• Exclusions: amputation, prostheses, or evidence of surgical alterations.
• Sexual Characteristics (genders)
• Examples: male + female, herm + female, or male + andromorph.
• Even if trans (lore) applies to one or more twins, TWYS is not overridden.
• Exclusions: One twin is visibly trans or if their breasts or genital configuration have otherwise been altered or removed through surgical or other means.

"Mirror Twins", Twins who appear to be reflectionally symmetrical, can be distinguished by their features and are considered half-identical unless one twin's mirror image differs from their twin in greater than one of the above traits.
Differences of all twins in more than just one of the above traits should normally be considered fraternal twins.

See also

• twins (lore): Tag for characters that are twins, regardless of how much DNA they share.
• fraternal twins: Tag for characters that share 50% of their DNA.
• identical twins: Tag for characters that share 100% of their DNA.
• Semi-identical (sesquizygotic) twins on Wikipedia: Information about Sesquizygotic twins.