An construction line is a line through a given point, oriented at a specified angle in threedimensional space and extending to infinity in both directions. Because construction lines extend to infinity, they are not calculated as part of the drawing extents.
The default method for drawing an construction line is to select a point along the line and then specify the direction of the line. You can draw an construction line in any of the following ways:
Horizontal draws the construction line parallel to the x axis of the current UCS.
Vertical draws the construction line parallel to the y axis of the current UCS.
Angle draws the construction line parallel to a specified angle.
Bisect draws the construction line perpendicular to an existing object.
Parallel draws the construction line parallel to an existing object.
To draw an construction line
Choose Draw > construction line.
Specify a point along the line.
Specify the direction.
To complete the command, press Enter.
Command line XLINE
Point along the infinite line (A) and the direction (B)
You can also draw construction line at a specific angle or at an angle relative to an existing object.
LINE: Creates straight line segments
RAY: Creates a semi-infinite line
XLINE: Creates an infinite line
System Variables Reference