There are different ways to classify or describe, or put into categories different map projections which can be helpful to kind of imagine, how they recreated or what properties they may have. There's these different methods of doing this. One way is based on the projection class, which refers to this thing called a developable surface. The most common projection classes, and certainly the ones I think that are most helpful to focus on when you're first starting out are the cylindrical class, the conic class, and the planar class. So, what are we looking at here? These are just the ways that a sheet of paper can be manipulated or used to transfer points from our 3D globe to our 2D map. The cylindrical one, here is one that's easy to kind of visualize. You've got your sheet of paper that's been wrapped around the globe. So, you'll notice that, it actually touches the globe, right here at the equator. If we unwrap that sheet of paper, you'll see that it's a rectangle. You can see from this diagram here, that this is the equator here and this is where the sheet of paper is touching the globe. We can also use the sheet of paper in a different way. We can have it shaped as a cone. That's sitting on top of the Earth sort of like a hat. Then if you unwrap that cone, you'll end up with a shape like this. So, when you start to see these maps of the world, you can start to visualize. I wonder what kind of a developable surface was used for this, it's hard to say sometimes. You'll notice that the sheet of paper touches the globe at a different location is not at the equator, it's a little bit farther north. That's a key point that we'll come back to is that, where the sheet of paper touches the globe, there is no distortion. That's something that we would like to have, is to have the least amount of distortion as possible or to know where that distortion is taking place. So, here, that's where we get a conic projection or we can have a sheet of paper. That if you want to think of it more like a shape like this, is that the paper is sort of lying flat on top of the globe. In this case, it's touching at the North Pole. Now it could touch at any location we want, but a common one. One is very popular is for to touch at the North Pole or the South Pole. So, now it's not actually touching along a line, like it is with this one or this one, it's actually touching at a point. When you project the earth onto that sheet of paper, you end up with a circular version of the Earth. So, if you see a circle like that, it indicates that it's probably a planar projection. If you see a rectangle, it's a cylindrical projection, and if you see this kind of wedge shape, it means it's a conic projection. These are not by any means, all of the different types of developable surfaces that are available. There's all kinds of weird and wacky methods that people have used to create projections. But these are by far, the ones that are most common, especially when you're first starting out. They're ones that you'll probably see very frequently. They're a good way to kind of start to visualize or think about how we take our 3D version of the world and turn it into a 2D map. So, these are developable surfaces and there are groups of projections that fall into each of these categories. So, there isn't just one planar projection, there's lots of variations of that. So, that's a one class or if you want to think of as a category of protection. So, there's different types of projections that are all based on this idea that there are planar. There are another group or category, or class of projections that are all based on this idea of using a cylinder, and there's other ones for conic. So, think of it that way. This is a way of describing projections based on how they were created, and if you want to visualize, it has a sheet of paper in relation to that reference globe.