Mirror twins are a subset of identical twins, in which the the twins are identical except that they are mirror images; one is left handed, the other right handed. If one has a birthmark* on eir right leg, the other will probably have it on eir left. The hair whorls in opposite directions, and their fingerprints are reversed. Sometimes, although not always, this holds to the extent that the internal organs, such as the heart, are also mirrored.

About 25% of identical twins will have some mirror traits; this is caused when the zygote splits into twins comparatively late in development. I don't have any sources that give a deadline, but apparently splitting as late as seven days after fertilization is enough to cause mirroring. As you might expect, conjoined twins are almost always mirror twins.

* Depending on the birthmark; many birthmarks are not genetically influenced.

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