Here's why:

* Transition metals are found in groups 3-12 of the periodic table. They have partially filled d orbitals.

* Inner transition metals are the lanthanides (elements 57-71) and actinides (elements 89-103). They have partially filled f orbitals.

* Post-transition metals are found in groups 13-16, and they have completely filled d orbitals.

Lead (Pb) belongs to group 14, making it a post-transition metal.