The twin composition plane in graphite is a 20o tilt boundary between lattices which are rotated, relatively, about an axis In the basal plane. Previous work has led to the proposition that some special type of structure must necessarily exist in the neighbourhood of the boundary which violates normal hexagon arrangement of the carbon atoms. It is demonstrated that tilt boundary of the required form can be explained as an array of partial dislocations, such a boundary being possible in either the hexagonal or the rhombohedral form. A boundary of this type is mobile, and can, by its movement, introduce or eliminate stacking faults and thus change the volume rhombohedral graphite present in the normal hexagonal lattice. Such effects have been reported previously. The true twinning plane in this model is not the composition plane, which is the plane {1101} referred to the structural (not the morphological) axes, but the plane {1121}