A solid ink stick comprising: a solid ink body having a length, a width, a height, and two ends; a first surface having a shape of a first polygon, each side of the first polygon including a first identifier (120, 320) and a second identifier (140, 340), each of the first identifiers on the sides of the first polygon being identical to one another and the first identifiers on the sides of the first polygon being in the same relative position on each respective side, and each of the second identifiers on the sides of the first polygon being identical to one another and the second identifiers on the sides of the first polygon being in the same relative position on each respective side; a second surface having a shape of a second polygon, the second polygon having the shape of the first polygon and being located on a face of the solid ink body that is opposite the first surface, each side of the second polygon including the first identifier and the second identifier, each of the first identifiers on the sides of the second polygon being identical to one another and the first identifiers on the sides of the second polygon being in the same relative position on each respective side of the second polygon, and each of the second identifiers on the sides of the second polygon being identical to one another and the second identifiers on the sides of the second polygon being in the same position on each respective side of the second polygon, wherein the first and second identifiers differ from each other and are arranged on the sides of each of the first and second polygon such that the solid ink body has two-fold rotational symmetry about an axis passing through its centre and equidistant from and parallel to the first and second surfaces, and wherein the first and second identifiers are each at least one indentation in and/or at least one protrusion from the solid ink body, each of which has a surface (128A, 148A, 148B, 328A, 348A, 348B) configured to reflect light. a solid ink body having a length, a width, a height, and two ends; a first surface having a shape of a first polygon, each side of the first polygon including a first identifier (120, 320) and a second identifier (140, 340), each of the first identifiers on the sides of the first polygon being identical to one another and the first identifiers on the sides of the first polygon being in the same relative position on each respective side, and each of the second identifiers on the sides of the first polygon being identical to one another and the second identifiers on the sides of the first polygon being in the same relative position on each respective side; a second surface having a shape of a second polygon, the second polygon having the shape of the first polygon and being located on a face of the solid ink body that is opposite the first surface, each side of the second polygon including the first identifier and the second identifier, each of the first identifiers on the sides of the second polygon being identical to one another and the first identifiers on the sides of the second polygon being in the same relative position on each respective side of the second polygon, and each of the second identifiers on the sides of the second polygon being identical to one another and the second identifiers on the sides of the second polygon being in the same position on each respective side of the second polygon, wherein the first and second identifiers differ from each other and are arranged on the sides of each of the first and second polygon such that the solid ink body has two-fold rotational symmetry about an axis passing through its centre and equidistant from and parallel to the first and second surfaces, and wherein the first and second identifiers are each at least one indentation in and/or at least one protrusion from the solid ink body, each of which has a surface (128A, 148A, 148B, 328A, 348A, 348B) configured to reflect light.