In laying out geometrical stairs the steps are arranged on the same principles as those above described.

The well hole in the centre is first laid down and the steps arranged round it.

In circular stairs with an open well hole, as in Fig. 213, the handrail being on the inside, the width of tread proportioned to the rise of steps should be set off along the dotted line, 18 inches in from the handrail, for the reasons above mentioned.

In stairs with a newel, such as that in Fig. 209, when the handrail, if any, might be on the outside, it will be sufficient if the steps have the proper width of tread in the centre of their length.

When laying out stairs practically on the building itself, the height to be gained is carefully marked out upon a "story rod!' on which are marked divisions corresponding in number and height to the risers: a similar rod is marked so as to show the treads; and from these rods the steps should be carefully marked upon the walls of the staircase.

A rod should also be prepared having marked upon it the exact width of the staircase, the length of steps, the position and size of newels, and also the size of the wall and outer strings, showing the thickness and the depth of the housings.