In general, a geometric lattice consists of an infinite, regular array of vertices in space, which can be modelled vectorially as a Bravais lattice. Some lattices may be skew, which means that their primary lines may not necessarily be at right angles. A reciprocal lattice is the periodic set of the wave vectors formula_24 in reciprocal space that make up the Fourier series of any function formula_25 whose periodicity is compatible with that of an initial direct lattice in real space. Equivalently, a wavevector is a vertex of the reciprocal lattice if it corresponds to a plane wave in real space whose phase at any given time is the same at every direct lattice vertex.