Subject: Drawing construction lines

A construction line is a special type of infinite line that is typically used when quick drafting techniques are required, for example, to make projections of front and side views.

Similar to an infinite line, a construction line is a line through a given point, oriented at a specified angle in three-dimensional space and extending to infinity in both direc­tions. Because construction lines extend to infinity, they are not calculated as part of the drawing extents.  

Construction lines are drawn automatically on their own layer, which by default is named CLINE. This makes it easy to change the appearance of construction lines; simply change the properties of the layer such as its color, transparency, etc. Addi­tionally, you can specify a different layer for locating construction lines using the CLINELAYER system variable.

You can easily draw construction lines in any of the following ways:

•   Select a point along the construction line and then specify the direction of the line (default drawing method).

•   Horizontally, vertically, or both.

•   By bisecting an existing point, an entity, or two entities.

•   According to a specified angle.

•   Parallel to or on top of an existing entity.

Construction lines are drawn to match the current UCS.
You can quickly rotate the UCS using Draw > Construction Line > Rotate UCS Z and Rotate UCS World (also available on the Construction Lines toolbar).

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Drawing construction lines by specifying a basepoint

Drawing construction lines by bisecting an angle or entity

Drawing construction lines horizontally and vertically

Drawing construction lines by specifying the angle

Drawing construction lines that parallel other entities

Erasing construction lines