The term "Bragg plane" is often associated with the field of crystallography and X-ray diffraction. It refers to a specific plane in a crystal lattice where constructive interference of X-rays occurs due to diffraction. When X-rays are scattered by the electron clouds of atoms in a crystal, the scattered waves can interfere with each other.
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For free-electron dispersion proportional to , states at and are degenerate on this plane, obtained by equating their squared norms. A nonzero periodic-potential Fourier component can mix them and open an avoided crossing in the nearly-free electron model. First-zone boundaries are selected planes of this kind.