Hold a short piece of the chair rail so that the rabbet on the bottom rests squarely on the end of each cut plank of paneling stock. Use a pencil to mark the bottom edge against the plank. The 1/2 -inch "invisible" margin now marked at the top of each cut plank will be your target for placing the nails out of sight. Once you install the chair rail, the nails will be under the rabbet.