The terms plesiomorphy and apomorphy are relative; their application depends on the position of a group within a tree. For example, when trying to decide whether the tetrapods form a clade, an important question is whether having four limbs is a synapomorphy of the earliest taxa to be included within Tetrapoda: did all the earliest members of the Tetrapoda inherit four limbs from a common ancestor, whereas all other vertebrates did not, or at least not homologously?
Plesiomorphy and apomorphy are relative terms; their application depends on the position of a group within a phylogenetic tree. For instance, when determining whether tetrapods form a clade, an essential question is whether having four limbs is a synapomorphy of the earliest taxa included in Tetrapoda: did all the earliest members of Tetrapoda inherit four limbs from a common ancestor, whereas other vertebrates did not, or at least not homologously?