A digon is a polygon having two sides (edges) and two vertices in geometry. In a Euclidean plane, its construction is degenerate because either the two sides must coincide or one or both must be curved. It can, however, be easily seen in **elliptic space**. A square, 4, is a truncated digon, t2. A triangle, 3, is a non-degenerate digon, d2.

A digon is thus a figure consisting of a pair of angles that are equal in size and opposite in direction. The name comes from these two features being the only parts of the figure that touch each other.

There are several ways to describe a digon. It can be said to have **two angle pairs** equal in size and opposite in direction. It can also be described as a quadrilateraltetrahedron, a four-faced cube with all faces equal in size and opposite in orientation. This last description makes clear that a digon is a degenerate case of a tetrahedron.

The first thing to note about digons is that there are only seven types of them, since they are all symmetric with respect to both horizontal and vertical axes. They can be divided into **two groups**: those with one positive and one negative angle (a trigonal digon), and those with an even number of **positive angles** (a tetragonal digon).

Trigonal digons consist of **three angles** that are equally spaced around a central point. They can be thought of as triangles with three legs instead of four.

The length and breadth of two-dimensional (2D) forms are the only dimensions. They are simple to draw on a piece of paper. A polygon is a two-dimensional form with **straight sides** that meet without gaps. A rectangle is a special type of polygon with **four straight sides**.

A circle is the most common example used to explain what a 2D shape is. It is a plane figure whose points can be joined by lines which are perpendicular to the drawing surface. The term "dimension" means size or scope, so a line can be described as having one dimension, while a plane figure like a circle or cube has two dimensions.

3D shapes have all three dimensions: length, width, and height. 4D shapes have all four dimensions.

Two-dimensional drawings contain information about the outline and areas inside the outline. They show the general shape of objects but not their exact details. Three-dimensional drawings show both the general shape and specific features of objects. Four-dimensional drawings would include **detailed drawings** of every part of an object.

As you can see, there are many different types of drawings that convey different amounts of information about **the subject matter**. It is important for artists to understand **these differences** because they need different tools to capture the detail required by each type of drawing.

2D Forms A 2D form is a shape or figure that has a length and a width. In other words, a flat object with simply length and width is only two dimensional. The sides of this form are made up of straight or curved lines. Planes, circles, squares, and rectangles are examples of **2D forms**.

3D Shapes A 3D form is one that has a length, a width, and a height. That is, it has **three dimensions**. Examples of **3D shapes** include cubes, pyramids, and spheres. A 2D image is also a 3D shape because you can see all around it. However, it doesn't have a length, a width, and a height - it's just two dimensions.

Shapes With One Dimension Less Than Two Are Only Valid For Use As Surfaces Unless Otherwise Specified: A surface is any two-dimensional shape that you can see from both above and below it. Examples of surfaces include planes, cylinders, and cones. A cylinder is a surface with a length but no width. A cone is a surface with a vertex at the top but no bottom. Other examples include paraboloids and torus. These kinds of surfaces are useful in creating objects such as jars or bottles that don't necessarily need a full dimension on each side.