We have measured polarization-dependent x-ray-absorption spectra (XAS) at the Cu K edge in magnetically-aligned powder samples of

{La}_{2}

{CuO}_{4}

{Nd}_{2}

{CuO}_{4}

{Bi}_{2}

{CuO}_{4}

{Ca}_{0.85}

{Sr}_{0.15}

{CuO}_{2}

, and

{SrCuO}_{2}

, all of which contain Cu coordinated by four coplanar O nearest neighbors. We have also measured Cu 2

p_{3 / 2}

x-ray photoelectron spectra (XPS) for all except the last compound. The measurements have been analyzed in terms of unrestricted Hartree-Fock calculations for

{CuO}_{6}^{10 -}

and

{CuO}_{12}^{22 -}

clusters, with energies and Coulomb parameters, including interactions with the core hole, for the Cu 3

d_{x}^{2}

y^{2}

, 4s, and 4p orbitals determined in an ab initio fashion. It is well known that the intensity ratio of the two XPS peaks is sensitive to the degree of hybridization between Cu 3

d_{x}^{2}

y^{2}

and O 2

p_{σ}

orbitals in the ground state. We show both by empirical correlations and model calculations that the relative shift of the continuum absorption threshold for ε^∥z (where z is perpendicular to the plane of the

{CuO}_{4}

units) and the shift of the main (3

d^{10}

L) XPS peak are also useful measures of the covalency. The Cu 4

p_{z}

-like quasibound state observed in XAS is sensitive as well to 4

p_{z -}

O 2p hybridization. In the cluster calculations, good agreement with the experimental features is obtained with use of a single adjustable parameter, the energy of the O 2

p_{σ}

level (measured relative to Cu 3

d_{x}^{2}

y^{2}

). We show that this energy varies linearly with the difference in Madelung energies for the O and Cu sites.

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