40. The location of the line relative to the planes of the projections.

With respect to the planes of the projections, the straight line can occupy a different position. A straight line that is not parallel to any of the main projection planes (see fig. 69) is called a straight line in general position. A straight line, parallel or perpendicular to one of the planes of the projections, is called a straight line of particular position.

Straight lines parallel to one of the projection planes are called level lines. Their name depends on which plane they are parallel. The straight line parallel to the horizontal plane of the projections is called the horizontal line and is denoted h in the drawings (Fig. 70).

The straight line parallel to the frontal plane of the projections is called the frontal and denoted f (Fig.71).

Fig. 70

Fig. 71

Fig. 72

The straight line parallel to the profile plane of the projections is called the profile plane and is designated p (Fig. 72).

For a straight level, one projection is parallel to the straight line itself and determines the angles of inclination of this straight line to two other planes of projections.

The parallelism of one of the planes of the projections determines the location of the other two projections of the direct level:

h ₂ || P ₂ / P ₁ ;

h ₃ _ | _ P ₂ / P ₃ ;

f ₂ || P ₂ / P ₁ ;

f ₃ _ | _ P ₂ / P ₃ ;

p ₁ _ | _ P ₂ / P ₁ ;

p ₂ _ | _ P ₂ / P ₁ ;

The straight lines h ₂ and f _{1 are} perpendicular to the vertical communication lines; p ₁ and p ₂ are located on the same vertical communication line and, when a two-projection drawing, must be defined by two points of a straight line p.

Straight lines, perpendicular to one of the projection planes, are called projectors. These lines, being perpendicular to one plane of projections, turn out to be parallel to two other planes of projections. Therefore, for projecting straight lines, one projection turns into a point, and the other two projections are parallel to

Fig. 73

direct and coincide in the drawing with the direction of the communication line (Fig. 73). There are horizontally projecting straight lines (AV), frontally projecting straight lines (CD) and profile projecting straight lines (EF).