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- [Voiceover] What I wanna[br]talk about in this video

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is shapes with four sides,

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and the mathy word for[br]shapes with four sides

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is quadrilateral.

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Quadrilateral.

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And any time you see[br]quad as part of a word,

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it's a good, it's a good sign

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that you're dealing with[br]the number four somehow.

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So a quadrilateral is[br]something that has four sides.

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So that is a quadrilateral,

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this is a quadrilateral,

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this is a quadrilateral.

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They all have four sides.

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This is a quadrilateral.

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even this is a,

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let me make it look even weirder,

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even this is a quadrilateral.

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So what's not a quadrilateral?

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Well, a triangle would[br]not be a quadrilateral.

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It has three sides,

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one, two, three.

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So let's cross that out.

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A pentagon that has five sides,

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that would not be a quadrilateral.

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It has one, two, three, four, five sides.

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A circle that has, I guess[br]you could say, no sides,

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it's just one big curve, it's a circle.

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That's not going to be a quadrilateral.

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If you had six sides, seven[br]sides, a hundred sides,

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none of that is going[br]to be a quadrilateral.

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So now let's think about

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the different types of quadrilaterals,

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or the different categories[br]of quadrilaterals.

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So one is the parallelogram.

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So a parallelogram is a quadrilateral,

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and as we learn more math,

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we'll learn other ways[br]of thinking about this.

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It's a quadrilateral where[br]opposite sides are parallel.

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And parallel is just another way of saying

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that they're going in the same direction.

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So what do I mean by that?

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So something like this,

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something like this,

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would be a parallelogram.

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Why?

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Because this side is[br]opposite to this side,

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and they're pointed in the same direction.

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They're going,

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they're going, if I were to draw an arrow,

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if I were to draw an arrow here,

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those arrows are pointed in the same way.

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So those two sides are pointed,

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are parallel, is the word we say.

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And these two sides,

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these two sides right[br]over here are parallel.

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So this is a parallelogram.

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So what are other[br]examples of parallelogram?

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Well even your classic square,

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even your classic square,

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is a parallelogram.

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And we'll talk more about[br]what makes a square special.

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It's a special type of parallelogram

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because this side

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is going in the same[br]direction as that side,

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and this side, and this side,

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whoops, let me do that in yellow,

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and this side is parallel to that side.

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So what's not a parallelogram?

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Well, something like,

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something like this

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would not be a parallelogram.

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You might say, "Wait,

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"I see two opposite sides are parallel."

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You might say, "Look,[br]this is parallel to this."

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But then you would see that[br]this is not parallel to this.

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One way to think about some[br]things that are not parallel

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is if the lines kept going,

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they would cross each other at some point,

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while these lines,

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these lines right over here,

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they're never going to cross each other.

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So this one right over here

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is not a parallelogram.

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It has one set of opposite[br]sides being parallel,

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but not the other.

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Another example of something[br]that is not a parallelogram

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would be this one right over here,

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because none of the[br]opposite sides are parallel.

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So parallelogram, opposite[br]sides are parallel.

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So now let's talk about,

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let's talk about more[br]types of four sided shapes,

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or quadrilaterals.

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So the next one we'll[br]talk about is the rhombus.

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So the rhombus is a type of parallelogram.

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The opposite sides need to be parallel,

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but that's not, by itself,[br]that doesn't make it a rhombus.

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The opposite sides need to be parallel,

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and all the sides have to be equal.

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So for example,

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this that I'm drawing,

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that is a parallelogram,[br]but it is not a rhombus.

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It's a parallelogram because that side,

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these opposite sides are parallel.

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If they kept going, they[br]would not cross each other.

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And these two opposite sides are parallel.

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So it's a parallelogram,[br]but it's not a rhombus

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because the blue sides are[br]longer than the yellow sides.

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So that is not a rhombus.

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A rhombus would have to look like this.

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A rhombus would have to look like that.

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So opposite sides are parallel

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and all the sides are the same length.

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And now you might say, "Well[br]maybe a square is a rhombus,"

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and I want you to think about that.

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Is a square a rhombus?

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Are all the sides the same length,

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and are the opposite sides parallel?

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Well we already said[br]that the opposite sides

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of a square are parallel.

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A square is a parallelogram.

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And all the sides of a[br]square are the same length.

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So a square is a rhombus.

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So one way to think about[br]rhombuses, or rhombi,

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is they're squares,

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and you could kind of view them as

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kind of a pushed over version of squares,

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if a square was moving

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really, really, really fast in a cartoon,

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that's what my brain thinks[br]of when I think of a rhombus.

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So now let's think of rectangle.

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And you might have heard the[br]word rectangle in the past,

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but let's think a little bit more

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about what makes a rectangle.

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So a rectangle is going[br]to be a parallelogram,

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but that by itself does[br]not make it a rectangle.

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So for example, this right[br]over here is a rectangle.

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Why is it?

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Well it's definitely a parallelogram.

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This side and that side are parallel.

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They'll never intersect.

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And this side and this side are parallel.

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They're never, they're[br]never going to intersect

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if you kept going on and on and on,

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or they're never going[br]to cross each other.

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But what makes it a rectangle?

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It's definitely a parallelogram,

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but what makes it, why do[br]we use the word rectangle?

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Well one way to think about it is

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they way that they come[br]together at the corners.

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So in a rectangle, the things come in to,

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I guess you could call[br]them square corners.

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And that's called a right angle.

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That's called a right[br]angle right over there.

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So this is what makes a rectangle.

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It's a parallelogram where all[br]the corners are right angles.

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You could put a little square there,

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if you wanna think about it that way.

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So for example, this right over here

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would not be a rectangle.

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Why?

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Cause you can't, if you[br]put a square here, notice.

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That does not, a square[br]doesn't fit in the corners

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the way that it fits right over here.

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That you have, the square[br]does not fit over here.

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This is a parallelogram,[br]but not a rectangle.

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A rectangle is a parallelogram[br]that has square corners.

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But what about our square?

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Is a square a rectangle?

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Well let's draw it out.

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Well let's think about it.

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A square, opposite sides are parallel.

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We've already said it's a parallelogram.

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And a square has,

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the corners are square.

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That's where when people[br]say make a square corner,

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that's where it comes from.

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The corners are square.

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They are at right angles.

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So the square is a rectangle.

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So the square is a really[br]interesting quadrilateral,

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because a square falls in[br]to all of these categories.

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The square, a square is a square,

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it's a rhombus,

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it's the type of rhombus

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where the corners are right angles,

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or you could say where[br]the corners are square.

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This one right over here is not a square,

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this one is square.

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They're both rhombuses,[br]or they're both rhombi.

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A square is also a rectangle.

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It's a parallelogram where[br]the corners are right angles,

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where they are square.

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A square is definitely a parallelogram,

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and everything we've talked[br]about is a quadrilateral.