It reduces to its simplest form when the path length difference for two initially parallel wave vectors is an integer multiple of the incident wavelength.  
This law applies whenever the reciprocal lattice points of a grid touch the surface of Ewald's sphere, since the radius of the sphere is the frequency.  It may be used to model its namesake type of Brillouin scattering, which applies to acoustic vibrations that provide the surfaces upon which it depends.  It reduces to its simplest form when the path length difference for two initially parallel wave vectors is an integer multiple of the incident wavelength.  FTP, name this equation that predicts the interference extrema for photon scattering from a periodic lattice, and that may be stated "n lambda equals 2d sine theta".
This law applies whenever the reciprocal lattice points of a grid touch the surface of Ewald's sphere, since the radius of the sphere is the frequency.  
{Bragg's law} (or {Bragg's equation} or {Bragg condition} of order n, etc.)