 We're going to be talking about the properties of parallelograms, and we'll start with the definition of a parallelogram. Parallelogram is a quadrilateral with both pairs of opposite sides parallel. A good idea to show on your diagram if it's not already marked that A, B, parallel to D, C, put your parallel line mark there, and then to differentiate the other side with the two arrow marks to show that those two lines or two segments are parallel to each other. We're going to be filling in a lot of information about different quadrilaterals, so today we'll be adding some information into our parallelogram piece of the quadrilateral family tree. You might want to get that out. Keep in mind that we're going to be putting a lot of things in here, so you might want to write small. We'll start by about the opposite angles of parallelograms. Opposite angles are congruent. And when we say opposite angles, we have two sets of opposite angles. We know that B and D are opposite each other and therefore congruent, and then separately A and C are opposite each other and congruent. And again, we want to differentiate that we have two marks here versus the one mark because all four are not congruent. Just the opposite angles are going to be congruent, so you want to show that differentiation again. The second piece is talking about the sides. Opposite sides are congruent and parallel. We've already marked on our diagram that the opposite sides are parallel, but now we can also mark that the opposite sides, as well as being parallel, parallel are also congruent. So A, D is congruent to B, C, and A, B is congruent to D, C. Again, not all four pieces congruent to each other, just the opposite sides. The third piece here talks about consecutive angles and consecutive angles again are going to be any angle set that are next to each other that are adjacent. So D and C are consecutive, B and C are consecutive, A and B are consecutive, and as you guessed, A and D are also consecutive. We have four sets of consecutive angles. Important to remember, they're supplementary, not congruent. That means together they're going to add up to 180 degrees. So, for instance, if I am told that angle C is equal to 100, because consecutive angles are supplementary, I know that angle D would have to be equal to 80, and actually that's what we'll be doing in the next video. Go ahead and add these pieces to your quadrilateral family tree. I have three pieces, but actually they're four separate statements. Opposite sides parallel, opposite sides congruent, opposite angles congruent, and consecutive angles are supplementary. Notice that I left a little space here because we will be adding a few more properties to the parallelogram as we go along.