Vertical angles are angles that share a common vertex
and whose sides form opposite rays.
In the image attached, I have drawn vertical angles.
Notice that the sides of the vertical angles 1 and 2 are the
opposite rays XA and XB and the opposite rays XC and XD.
Angles AXD and BXC would also be vertical angles.
Also note that when 2 lines intersect, they will
always form 2 pairs of vertical angles.