 Let's say you're at a building at this corner in a city and you want to get a package to someone at this corner. If you're sending it by bike messenger, you tell them to go three blocks east and four blocks north. But if you're sending the package by drone you don't care about streets. Instead, you fly the drone five city block lengths at a bearing of 46.9 degrees. The buildings haven't changed position, but depending on the method of transportation we specify the location with one set of numbers for a bicycle and another set of numbers for a drone. Similarly, when we have points on a graph we can use a city block grid to locate the point. The east-west direction is called the x-axis and the north-south direction is called the y-axis. We specify a point's location by its distance from the center on the x-axis, its x-coordinate, and the distance on the y-axis, its y-coordinate. We read the coordinates of this point as 3, 4. These are called Cartesian coordinates. They're named after René Descartes, the French philosopher and mathematician who published the idea in 1637. But there's no law that says that's the only way to specify the location of a point. Similar to the way we use distance in bearing with a drone we can specify a point's location by its distance from the center and its angle measured counterclockwise from zero degrees where zero degrees is due east. The first number is referred to as the radius and the second number is the angle, often referred to by the Greek letter theta. These are called polar coordinates. You can think of the circular grid lines as what you might see if looking at the longitude and latitude lines on a globe from above the North Pole. Again, the points haven't moved. We're using different methods to specify the location. It's at 3, 4 in Cartesian coordinates and at radius 5, theta 53.13 degrees in polar coordinates. Most of the time people use Cartesian coordinates for graphs, especially when graphing equations that involve motion along the x-axis. Polar coordinates are useful when graphing results of circular motions with a changing angle and those are the two main systems of coordinates that mathematicians use. Cartesian coordinates and polar coordinates.