  We propose that cuprate superconductors are in the vicinity of a spontaneous
d-wave type Fermi surface symmetry breaking, often called a d-wave Pomeranchuk
instability. This idea is explored by means of a comprehensive study of
magnetic excitations within the slave-boson mean-field theory of the t-J model.
We can naturally understand the pronounced xy anisotropy of magnetic
excitations in untwinned YBa_{2}Cu_{3}O_{y} and the sizable change of
incommensurability of magnetic excitations at the transition temperature to the
low-temperature tetragonal lattice structure in La_{2-x}Ba_{x}CuO_{4}. In
addition, the present theoretical framework allows the understanding of the
similarities and differences of magnetic excitations in Y-based and La-based
cuprates.
