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4.7 PARALLEL PLANES

Two planes are parallel if two intersecting lines in one plane are parallel to two intersecting
lines of other plane (Fig. 4.20).

Figure 4.20

If two planes are parallel their respective traces are parallel. If two parallel planes cut by a
third plane, the lines of intersection are parallel (Fig. 4.21 -58).

Figure 4.21

If two respective traces are parallel the respective principal lines are parallel too.

4.8 INTERSECTING PLANES

The line of intersection of two planes is a line common to both planes; therefore its horizontal
piercing point lies in the horizontal trace of each plane, hence at their point of intersection. The
frontal piercing point of the line of intersection lies at the intersection of frontal traces. The line
joining these two piercing points is the required line of intersection (Fig. 4.22).

l 2

l 1

Figure 4.22

Special case of line of intersection of planes
Either pair of traces does not intersect on limits of drawing.
Draw an auxiliary plane parallel to the plane of projections. This plane will cut from the given
planes, lines which will intersect in a point of the required line of intersection. A second such

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