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?
The terms plesiomorphy and apomorphy are context-dependent. Their application depends on the evolutionary lineage being studied. For example, when analyzing whether tetrapods form a clade, it's crucial to determine if having four limbs is a shared trait of the earliest tetrapod ancestors or a later derived characteristic.