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The true length of line AB may also be obtained by assuming the axis through point A and
perpendicular to the horizontal plane of projection (Fig. 5.10). Line AB is rotated parallel to the
frontal plane of projection and projects in true length in the front view. The angle α is the true angle
between AB and the horizontal plane of projection.
Figure 5.10
A straight line, when revolved about an axis perpendicular to Π 1, maintains always its original
angle with Π 1. If the axis is perpendicular to Π 2, the angle between the revolving line and Π 2 does
not change.
As in the revolution of a straight line, a plane, when revolved about an axis perpendicular to
Π 1, maintains always its original angle with Π 1. If the axis is perpendicular to Π 2, the angle between
the revolving plane and Π 2 does not change (Fig. 5.11).
Figure 5.11
A frontal-projecting plane (ABC) projects in its true shape when rotated into parallelism about
a frontal-projecting axis l through one point of the plane (point A) (Fig. 5.12).
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