The horizontal plane is usually chosen for the projection plane. All the points in this plane have elevation 0. The planes parallel with the projection plane are called levelplanes. All points in a levelplane have the same distance from the projection plane and therefore the same elevation. Specially, the planes with the integer elevation are called main levelplanes. 
Main levelplanes. 
The point A is 2m below and the point B is 3m above the projection plane. 
Graded straight line with the indicated interval. 
\( n_p = \tan\alpha = \frac{1}{i}\) 

Intersecting straight lines  the intersection point has the same elevation on both straight lines. 
Parallel straight lines  the projections are parallel and they have the same direction of the fall and same intervals. 
Skew straight lines  intersecting projections lines with different elevations of the intersection point on given lines. 
Skew straight lines  parallel projections with different intervals. 
Main horizontals, scale of slope and indicated interval of the plane. 
Main horizontals are the intersection of the plane and main levelplanes. 



Created by Sonja Gorjanc, translated by Helena Halas and Iva Kodrnja  3DGeomTeh  Developing project of the University of Zagreb