Variations of a line

In mathematics a line plays many roles -- as in this fine poem (which is a sonnet, more or less).

     Lines     by Martha Collins

     Draw a line. Write a line. There.
     Stay in line, hold the line, a glance
     between the lines is fine but don't
     turn corners, cross, cut in, go over
     or out, between two points of no
     return's a line of flight, between
     two points of view's a line of vision.