We argue that in cuprate physics there are two types, hole content per CuO_2 plane (P_<pl>) and the corresponding hole content per unit volume (P_<3D>), of hole-doping concentrations for addressing physical properties that are two-dimensional (2D) and three-dimensional (3D) in nature, respectively. We find that superconducting transition temperature (T_c) varies systematically with P_<3D> as a superconducting “dome” with a universal optimal hole-doping concentration P^<opt.>_<3D> = 1.6 × 10^<21> cm^<−3> for single-layer high temperature superconductors. We suggest that P^<opt.>_<3D> determines the upper bound of the electronic energy of underdoped single-layer high-T_c cuprates.
