 This video describes properties of a trapezoid. So it says a quadrilateral is a trapezoid if and only if it has exactly and we're going to fill in the blank. If you look at the picture you can see the little arrows. We know that means parallel and that's what makes a trapezoid a trapezoid. If it has exactly one pair of parallel sides. So here we see these are the sides that are parallel. Of course you can tell that these two sides are not parallel and so that's what makes this a trapezoid. A trapezoid can also be isosceles and a trapezoid is isosceles if and only if the legs, whoops, I need to my pen here, if the legs are congruent. Now in very very important to understand in this picture, which ones are the legs? So what you need to know is that the legs are the non parallel sides and we call the parallel sides the bases. So when we're talking about an isosceles triangle it is really important that you know that the parallel sides are called bases and the non parallel sides are called legs. So a trapezoid is isosceles if the legs are congruent. There are a couple other things about an isosceles trapezoid and that's what we're going to write down here. If a trapezoid is isosceles then another thing that's true is that the base angles are congruent. Now the base angles would be the two at the bottom. So those are congruent and then the two at the top. So notice that they're not all four congruent. The two bottom base angles are congruent and the two top base angles are congruent. And then the last thing about an isosceles trapezoid is that the diagonals are congruent. So in a regular trapezoid or in a normal trapezoid the diagonals might not be congruent but an isosceles trapezoid the diagonals are always congruent.