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3 STRAIGHT LINES INTO SYSTEM OF COORDINATE PLANES

A line is the path of a moving point, and is not necessarily straight. Yet in ordinary use, the
term line, by itself, and without anything to imply the contrary, always means a straight line.
The following statements are evident (reminding of aforesaid):
- the projection of a straight line is a straight line;
- projection of any point of the line lies on the projection of the line.
Since any point in space is definitely determined when its projection on Π 1 and Π 2 are known,
it follows then, in general, any two straight lines assumed at random, one in Π 1 and one in Π 2, are
generally the projections of one and the only one straight line in space.

3.1 LOCATION OF THE LINE

The straight line is given using two projections of two points.
The two projections of a line being given, the line is in general completely determined
(Fig.3.1).

Figure 3.1

Since the projections of a line are made up of the projections of all points of the line, the
projection of a line which passes through a given point, passes through the projections of the point.
Among the different positions in which a line may be placed concerning to the coordinate
planes, we limit our choice of location to one angle.
st
The positions in the 1 angle are shown in Fig. 3.2.

Figure 3.2

3.2 POSITION CLASSIFICATION

Particular position of the line
Line which parallel to one of projection planes is called level line: horizontal, frontal and
profile lines (Fig. 3.3).
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