• Transversal … a line that intersects 2 others.
(e.g.) The red line in Figure 1
• Vertical Angles
… The opposite angles formed by two intersecting straight lines.
(e.g.) a and d in Figure 1 (b and c, e and h, g and f in Figure 1 are also the vertical angles.)
• Corresponding Angles
… a pair of angles at the same location at each intersection.
(e.g.) a and e in Figure 1 (b and f, c and g, d and h in Figure 1 are also the corresponding angles.)
• Alternate Interior Angles
… a pair of angles inside the parallel lines on opposite sides of the transversal.
(e.g.) c and f, d and e in Figure 1 are the alternate interior angles.