Fig. 21.
Two of these angles are used in practice, the 1st and the 3rd; the most modern practice is to use the 3rd, although the 1st is still used in some manufacturing establishments and by some teachers.
Fig. 22.
We will take, first, the 1st angle, and compare it with the results obtained from the 3rd angle. We place the solid (exercise 1) in space in the 1st angle, and also place a similar one in the 3rd angle (see Figs. [23] and [24]).
Fig. 23.
