Figure 19(d) Extension at Top of Run. A 12 inch (305 mm) horizontal extension is shown. The extension starts at the nosing of the top tread and continues for at least 12 inches (305 mm) before the railing turns. This is also indicated as dimension X. The figure illustrates a handrail extension that returns to the support post with a lower horizontal segment less than 27 inches (685 mm)above the floor.