Plane [math]\alpha[/math] is given by points [color=#3c78d8][i]GHI[/i][/color] on the cube. Horizontal projection plane [math]\pi[/math] is [color=#3c78d8]blue face[/color] of a cube, frontal projection plane [math]\nu[/math] is [color=#e06666]red face[/color]. The base (ground) line[i] x12[/i] is intersection line of projection planes, i.e.[math]x12=\pi\cap\nu[/math]. Trace lines of the planes [i][color=#ff0000]n, p[/color][/i] are lines where the plane [math]\alpha[/math] meets the planes of projections, i.e. [math]p=\alpha\cap\pi[/math] and [math]n=\alpha\cap\nu[/math].[br][list][*]Horizontal line [i][color=#cc0000]h[/color][/i] of a plane is parallel to the blue face. [/*][*]The line of steepest descent [color=#1155Cc][i]s[/i][/color] is perpendicular to the horizontal line. This line determines the slope angle of the plane.[br][/*][/list]

Planes are aither vertical, horizontal, inclined or oblique. A [b]horizontal plane[/b] is any plane parallel with the ground plane [math]\pi[/math]. All points in horizontal plane are at the same height or elevation above [math]\pi[/math]. [br]A [b]vertical plane[/b] is defined as a plane that is perpendicular to the[color=#0B5394] horizontal (ground) projection plane[/color] [math]\pi[/math]. [br]A [b]inclined plane[/b] is defined as a plane that is perpendicular to [color=#ff0000]frontal projection plane[/color] [math]\nu[/math].

[list][*][b]Trace point[/b] of the line in the plane is always on the trace of the plane. [br][/*][*]If a line in a plane appears as a point, the plane appears as an edge.[/*][/list]