A vertex is a mathematical word for a corner. Most geometrical shapes, whether two or three dimensional, possess vertices. For instance, a square has four vertices, which are its four corners. A vertex can also refer to a point in an angle or in a graphical representation of an equation.
TL;DR (Too Long; Didn't Read)
In math and geometry, a vertex – the plural of vertex is vertices – is a point where two straight lines or edges intersect.
Vertices of Line Segments and Angles
In geometry, if two line segments intersect, the point where the two lines meet is called a vertex. This is true, regardless if the lines cross or meet at a corner. Because of this, angles also have vertices. An angle measures the relationship of two line segments, which are called rays and which meet at a specific point. Based on the above definition, you can see that this point is also a vertex.
Vertices of Two-Dimensional Shapes
A two-dimensional shape, such as a triangle, is composed of two parts – edges and vertices. The edges are the lines that make up the boundary of the shape. Each point where two straight edges intersect is a vertex. A triangle has three edges – its three sides. It also has three vertices, which are each corner where two edges meet.
You can also see from this definition that some two-dimensional shapes do not have any vertices. For example, circles and ovals are made from a single edge with no corners. Since there are no separate edges intersecting, these shapes have no vertices. A semi-circle also has no vertices, because the intersections on the semi-circle are between a curved line and a straight line, instead of two straight lines.
Vertices of Three-Dimensional Shapes
Vertices are also used to describe points in three-dimensional objects. Three-dimensional objects are composed of three different parts. Take a cube: each of its flat sides is called a face. Each line where two faces meet is called an edge. Each point where two or more edges meet is a vertex. A cube has six square faces, twelve straight edges, and eight vertices where three edges meet. In other words, each of the cube's corners is a vertex. As with two-dimensional objects, some three-dimensional objects – such as spheres – do not have any vertices because they do not have intersecting edges.
Vertex of a Parabola
Vertices are also used in algebra. A parabola is a graph of an equation that looks like a giant letter "U." The equations that produce parabolas are called quadratic equations, and are variations on the formula:
A parabola has a single vertex -- either at the bottom point of the "U," if the parabola opens upwards -- or at the top point of the "U," if the parabola opens downwards, like an upside down "U." For instance, the bottom point of the graph of the equation y = x2 is located at the point (0,0). The graph rises on either side of this point. So (0,0) is the vertex of the graph of y = x2.
About the Author
Jon Zamboni began writing professionally in 2010. He has previously written for The Spiritual Herald, an urban health care and religious issues newspaper based in New York City, and online music magazine eBurban. Zamboni has a Bachelor of Arts in religious studies from Wesleyan University.