Bragg plane
= Bragg plane
{c}
{title2=$\mathbf k\cdot\mathbf G=G^2/2$}
For free-electron dispersion proportional to $k^2$, states at $\mathbf k$ and $\mathbf k-\mathbf G$ 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.