For a more complete characterization of the shape of sedimentary particles, a method for tridimensional shape analysis is described. It is proposed to return to Wadell's original definition of sphericity, i.e., the degree of true sphericity, as the basis for the quantitative description of shape. However, instead of analyzing each grain form as such, the shape is approximated by a tetrakaidekahedron which is defined by three mutually perpendicular particle dimensions. Using these measured dimensions and the geometric characteristics of the tetrakaidekahedron, the shape of particles can be defined simultaneously in terms of Wadell's sphericity and Zingg's shape factor. An expression is derived for approximating the degree of true sphericity of sedimentary particles. It is shown that the use of this expression as the "working sphericity" seems to be justified and may be considered an improvement over other approximations. An example of the workability of the technique is given by the tridimensional shape analysis of the component grains of a sample of Woodbine sandstone. It is thought that the method described can be of use in many phases of stratigraphic and sedimentary research.