A four-dimensional object to represent the forces and moments in a three-dimensional structure. Graphic statics is a visual geometric method of describing forces in structures. It was developed in the 19th century by James Clerk Maxwell, but has previously been limited to structures which can carry only axial forces. However by using four-dimensional geometry it is possible to generalise the method for frame structures which carry forces and bending moments. The mathematical description of structural equilibrium that emerges bears similarities to that of electromagnetism, a fact that would no doubt have interested Maxwell. See ‘The geometry of structural equilibrium.’ R. Soc. open sci. 4: 160759. http://dx.doi.org/10.1098/rsos.160759.