We will determine the atomic and magnetic structure of two polymorphs of manganese cyanide, Mn(CN)2, which exhibit negative thermal expansion (NTE). The two forms are related; both are based on a diamond lattice formed from corner-sharing Mn(CN)4 tetrahedra. They differ in that the alpha form contains two independent interpenetrating networks, whereas the beta form contains a single network and voids. We will analyse both total scattering and Bragg diffraction at low and high temperature to obtain a complete description of the short and long range order, and gain an insight into the mechanism responsible for the NTE. The two forms show very different magnetic behaviour; the alpha form orders antiferromagnetically at about 65K, whilst it is reported that the beta form does not order. We will determine the magnetic structure of the alpha form, and measure its order parameters.