The space required for timber and plastering under the steps is about 5 inches for ordinary stairs, or 6 inches if furred; set a gauge, therefore, at 5 or 6 inches, as the case requires, and run it on the lower edge of the plank, as ab (Fig. 128). Commencing at one end, lay the longest side of the pitch-board against the gauge-mark, a b, as at c, and draw by the edges the lines for the first rise and tread; then place it successively as at d, e, and f, until the required number of risers shall be laid down. To insure accuracy, it is well to ascertain the theoretical raking length of the pitch-board by computation, as in note to Art. 536, by getting the square root of the sum of the squares of the rise and run, and using this by which to divide the line ab into equal parts.