See: PDF1.7 s188.8.131.52
The two segments are finished with butt caps and the space between the ends is filled with a triangle.
Specify as 2 or :bevel.
The outer lines of the two segments continue until they meet at an angle.
Specify as 0 or :miter.
An arc of a circle is drawn around the point where the segments meet.
Specify as 1 or :round.