Lines that extend to infinity in one or both directions, known as rays and construction lines, respectively, can be used as references for creating other objects.
A construction line (also known as an xline) can be placed anywhere in three-dimensional space. A ray is a line in three-dimensional space that starts at a point you specify and extends to infinity. Unlike construction lines, which extend in two directions, rays extend in only one direction. Using rays instead of construction lines can help reduce visual clutter. Both construction lines and rays are ignored by commands that display the drawing extents.
