**k-space ; reciprocal space** The reciprocal lattice of a lattice (usually a Bravais lattice) is the lattice in which the Fourier transform of the spatial wavefunction of the original lattice (or direct lattice) is represented. This space is also known as momentum space or less commonly k-space, due to the relationship between the Pontryagin duals momentum and position. The reciprocal lattice of a reciprocal lattice is the original or direct lattice.

If the direct space is spanned by vectors

**a1**,

**a2**,

**a3** its reciprocal space can be generated by the following three reciprocal primitive vectors:

**b1** = (

**a2** x

**a3**) 2π / V

**b2** = (

**a3** x

**a1**) 2π / V

**b3** = (

**a1** x

**a2**) 2π / V

V=

**a1** . (

**a2** x

**a3**)

__Source:__ http://en.wikipedia.org/wiki/Reciprocal_lattice