Extrusion apparatus is employed in a co-extrusion process to produce a triangular-shaped barrel for a writing instrument. An inner layer of material is extruded in tubular form having a substantially circular cross-section. A coating layer of three strips of material, equally spaced one from the other, is co-extruded about the outer periphery of the inner layer. A combination of forces produced during the process is effective to distort the inner layer to form a product having a substantially triangular shape.