  The bulge in the doping dependence of the basal plane area in hole doped
cuprate superconductors is connected with a non-double-occupancy constraint for
the oxygen cages in the CuO_2 lattice. This constraint favors the formation of
3a hole pairs growing to filaments with gapless excitations along the (pi, pi)
direction. Thus in the pseudogap regime a nodal metal of hole pairs is created.
Densest packed 3a hole pairs stabilize the optimum doped state at n =1/6 ~
0.16.
