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For the group being studied, a trait that arose prior to the common ancestor of that group. This trait will be shared with the outgroup and all descendants of the common ancestor, therefor the ancestral trait does not define the group being studied.
In this diagram assume vertebrates are the group being studied and that stick insects are the outgroup. The trait of bilateral symmetry is the ancestral trait (plesiomorphy). Notice how all taxa have this trait.
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