/ l
         /
        /
     C /  B
---------------- m
   A / D
    /
   /
  /
 /
/

A and B are vertical angles. C and D are vertical angles. This is contingent upon l and m being straight lines. Note that A and B have the same magnitude, and that C and D have equal magnitudes as well. This is because A + C = π, A + D = π, D + B = π, B + C = pi;, and in the nice world of Euclidean geometry, you can equate things that have the same value (A + C = A + D, eg) and then subtract off the common additives (C = D).