The set square
The rectangular 45-degrees triangle has the same measure designation as the drawing board. The two short sides have a measure designation of 10 centimeters, while the long side has a measure designation of 14 centimeters. An indication on the middle of the long side makes it easy to draw a perpendicular.
There are four holes in the set square for fixing it over the knobs. The round hole in the 90 degree corner fits around a knob.
By putting one of the short sides of the triangle aligned with the edge of the frame, the set square creates two angles on the board, one of 90 and one of 45 degrees.