Two-dimensional space

A two-dimensional space is a mathematical environment defined by two dimensions. In such a space, every point possesses two degrees of freedom. This means a point's location can be described using two independent coordinates. You can also think of this as having two independent directions in which to move. These spaces are often called planes or surfaces. They are not always meant to represent physical objects. Some spaces represent mathematical models or the configuration of physical systems.

The most basic version is the flat Euclidean plane. This is an idealization of a flat surface, like a chalkboard or a sheet of paper. In this space, any two points can be joined by a single unique straight line. You can also measure the distance between any two points along that line. A key feature of this plane is the behavior of parallel lines. If a third line crosses two other lines at right angles, those two lines are parallel. In a Euclidean plane, these lines stay at a uniform distance from each other and never intersect.

However, two-dimensional spaces can also be curved. A sphere or a cone provides an example of a curved surface. On a sphere, lines that appear locally parallel might eventually converge. In a hyperbolic plane, lines that start out parallel might diverge or move away from each other. Mathematicians call surfaces that have a locally Euclidean concept of distance but non-uniform curvature Riemannian surfaces. Some of these surfaces are embedded in a three-dimensional Euclidean space. For example, a cylinder or a cone is a ruled surface. This means it contains a straight line through every point.

Other surfaces have unique physical or mathematical properties. Minimal surfaces, such as soap films, locally minimize their area. There are also relativistic Lorentzian surfaces. These look locally like a two-dimensional slice of relativistic spacetime. This slice includes one spatial dimension and one time dimension. Examples include the flat Lorentzian plane, which is a subspace of Minkowski space. Other constant-curvature examples include the de Sitter and anti-de Sitter planes. These complex structures help describe the geometry of the universe.

Beyond geometry, some spaces are defined by different rules called non-Euclidean structures. An affine plane is a type of space that has a notion of parallel lines. However, an affine plane has no way to measure distance. In contrast, a projective plane does away with both distance and parallelism entirely. There are also topological surfaces. These can be stretched, twisted, or bent without changing their essential properties. Additionally, an algebraic surface is defined as a two-dimensional set of solutions to a system of polynomial equations.

Some mathematical spaces carry extra arithmetical structures. In a vector plane, points are called vectors. A vector plane is an affine plane that includes a special designated origin called a zero vector. You can add vectors together or scale them by a number. These spaces might also use a Euclidean, Lorentzian, or Galilean concept of distance. Other spaces, like the complex plane, use points that are actually numbers. In the complex plane, you can add and multiply these points. The hyperbolic number plane and the dual number plane work in similar ways.

Mathematicians often use numbers to define these spaces instead of geometric axioms. One fundamental example is the real coordinate space. This space consists of pairs of real-number coordinates. Other types of numbers can also serve as coordinates. For instance, the complex plane is two-dimensional when using real-number coordinates. However, it is considered one-dimensional when using complex-number coordinates. A two-dimensional complex space can be represented using four real dimensions. Finally, a two-dimensional lattice is an infinite grid of points. These points can be represented using integer coordinates.