 Lines, circles, and other curves in the plane can be expressed algebraically using Cartesian coordinates, named for its inventor Rene Descartes. These are defined with respect to a distinguished point in the plane called the origin, denoted \(O\). Each point is specified by how far it is to the right of and above \(O\), written as a pair of numbers. For example the pair (2.1, 3.56) specifies the point 2.1 units to the right of \(O\), measured horizontally, and 3.56 units above it, measured vertically; we call 2.1 the \(x\) coordinate and 3.56 the \(y\) coordinate of that point. Either coordinate can be negative: the pair \((-5, -1)\) corresponds to the point 5 units to the left of \(O\) and 1 unit below it. The point \(O\) itself is coordinatized as (0, 0).