Take a ruled sheet of paper. Draw (in thick colour) two parallel lines I and m. Draw a transversalt to the lines land m. Label Z1 and 22 as shown [Fig 5.28(i)].
Place a tracing paper over the figure drawn. Trace the lines 1, mandt.
Slide the tracing paper along t, until I coincides with m.
You find that 21 on the traced figure coincides with 22 of the original figure.