In cladistics, whenever three monophyletic groups of organisms are compared, two of them are more closely related to each other than either is to the third. The third group is known as the outgroup. The evolutionary conclusion from this is that the outgroup branched from the parent group before the other two groups branched from each other.

Some examples, with outgroup on the right:

• Humans, chimpanzees — gorillas
• Placental mammals, Marsupials — Monotremes
• Tetrapoda, Actinopterygii — Elasmobranchii
• Chordates, Echinoderms — Mollusks