`
/ `*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).