Scale of chords

Scale of chords is used to measure angles when a protractor is not available, by comparing the angles subtended by chords of an arc at the centre of the arc.
Draw a line AO of any suitable length.
At O, erect a perpendicular OB such that OB – OA
With O as centre, draw an arc AB
Divide the arc in to 9 equal parts  by the following method.

1. On arc AB, mark two arcs with centers A and B and radius – AO. By this the arc AB is divided in to three equal parts.
2. By trial and error method, divide each of these three parts in to three equal subdivisions.

The total length of AB is now divided in to 9  equal parts. Number the divisions as 10, 20, 30, 40 ,etc.
Transfer all the divisions on the arc to th line AO by drawing arcs with A as a centre and radii equal to the chords A-10, 10-20, 20-30, …. AB.
Construct the linear degree scale by drawing the rectangles below AC.  Mark the divisions in the rectangle with zero below A and number the divisions subsequently as 10^o, 20^o, 30^o, 40^o, ….., 90^o