  The electron hole concentration p in the CuO2 planes of YBa2Cu3O6+x (YBCO)
has been evaluated using the unit cell length in the c-direction together with
a simple estimation for p in the chain oxygen ordered phases. The empirical
relationship between p and c obtained allows calculation of p from the c-axis
lattice parameter for the entire doping range of YBCO. It is also suggested
that the empirical parabola describing the relationship between Tc and p in
La2-dSrdCuO4 is reasonably correct for YBCO except for the region around p
=1/8, where the Tc of YBCO is suppressed by as much as 17 K. A correction of
the parabola for the p = 1/8 effect is presented.
