This section is from the book "Elements Of Construction", by Charles A. King. Also available from Amazon: Elements of construction.
The Bevel (Fig. 9) may be set for use in marking and testing any angle, in the same manner that the try-square is used upon rectangular work. The illustration shows the bevel and the steel square in position for setting the bevel at an angle of 45°. It will be noticed that the blade of the bevel rests upon the same figures upon both the blade and the tongue of the square.

Fig. 9. - Bevel and Steel Square. The bevel is set at an angle of 45°.
 
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