Top Board
fits over the side-pieces, and the bottom boards fit between the side-pieces, it is evident that the bottom boards are shorter than the top board by just the width of the two side-pieces. Be careful to allow for this width at the ends, when you nail the boards together, as shown in Fig. 303.
Figs. 302 and 303.
After the holes are bored through the two boards, nail the top board and bottom boards in place, as shown in Fig. 301.
You must, of course, put the bottom board with the holes in it, on top of the bottom board without the holes. This will give two sockets, in which to rest and turn the ends of the rollers.