Pure monoclinic calcium pyrophosphate dihydrate (m‐CPPD) has been synthesized and characterized by synchrotron powder X‐ray diffraction and neutron diffraction. Rietveld refinement of complementary diffraction data has, for the first time, allowed the crystal structure of m‐CPPD to be solved. The monoclinic system P21/n was confirmed and unit‐cell parameters determined: a = 12.60842 (4), b = 9.24278 (4), c = 6.74885 (2) Å and β = 104.9916 (3)°. Neutron diffraction data especially have allowed the precise determination of the position of H atoms in the structure. The relationship between the m‐CPPD crystal structure and that of the triclinic calcium pyrophosphate dihydrate (t‐CPPD) phase as well as other pyrophosphate phases involving other divalent cations are discussed by considering the inflammatory potential of these phases and/or their involvement in different diseases. These original structural data represent a key step in the understanding of the mechanisms of crystal formation involved in different types of arthritis and to improve early detection of calcium pyrophosphate (CPP) phases in vivo.