A key topic that you're going to use for the rest of Geometry is Vertical Angles. The key things to remember about Vertical Angles is that they're formed by intersecting lines or intersecting line segments they share vertex. So over here our vertex is right there but they're on opposite sides of each other. And last the most important part, they're always congruent. You're going to use this in proofs later on, so we have 4 angles here formed by 2 intersecting lines.
What are the vertical angles? Well the vertical angles one pair would be 1 and 3. So I could say the measure of angle 1 is congruent to the measure of angle 3, they're on, they share this vertex and they're on opposite sides of it. The second pair is 2 and 4, so I can say that the measure of angle 2 must be congruent to the measure of angle 4. So vertical angles, common vertex, congruent and on opposite sides of each other.