Ever wonder what exactly a diagonal is? Don’t feel bad if the term seems a bit hazy – it’s one of those concepts we learned a long time ago in geometry class and never really thought much about since. But diagonals are actually quite useful in the real world, from the frames of kites to the beams in buildings to the angles of a diamond.

A diagonal is simply a line that connects two nonadjacent corners of a shape. It’s not horizontal, it’s not vertical – it cuts across at an angle. The word comes from Greek roots meaning “from angle to angle.” Diagonals are inclined lines that join opposite corners of squares, rectangles, polygons, and other geometric shapes.

So now you know – a diagonal is just a slanted line segment that spans the shape from corner to corner. Keep an eye out as you go about your day and you’ll start to notice diagonals everywhere, in architecture, art, engineering, and nature. Geometry may not have been your favorite subject, but its fundamental concepts like the diagonal shape the world around us.

What Is a Diagonal?

A diagonal is a straight line that connects two nonadjacent vertices of a polygon. In other words, it’s a line segment that joins two corners of a shape that are not next to each other.

To visualize a diagonal, think of a square. The square has four sides of equal length, and four corners or vertices. If you were to draw a line from one corner to the opposite corner, bypassing the sides in between, that line would be the diagonal. The diagonal cuts the square into two equal triangles.

Some key characteristics of a diagonal:

  1. It must pass through the interior of the polygon, not just connect vertices along the perimeter.
  2. A diagonal always connects nonconsecutive vertices – it skips over other vertices and sides.
  3. A diagonal divides the polygon into smaller polygons, often triangles. In the case of a square, it divides it into two triangles. For an octagon, it would divide it into four triangles.
  4. The length of a diagonal depends on the size and shape of the polygon. The diagonal of a square is longer than one of its sides, while the diagonal of a regular pentagon is shorter than its sides.
  5. A polygon can have multiple diagonals, as long as no two diagonals cross each other. The number of diagonals in a polygon depends on the number of sides it has.

In summary, a diagonal is a line segment that joins two non-adjacent vertices of a polygon, cutting through its interior. It’s a simple concept with many applications in mathematics and design. Now you know what a diagonal is and how to spot one!

Types of Diagonals in Geometry

There are a few types of diagonals you’ll come across in geometry.

Line segments

The most basic diagonal is a line segment that connects two non-adjacent vertices of a polygon. For example, in a square, a diagonal line segment would connect two opposite corners. These diagonals are important – they can bisect the square into two congruent triangles.


Diagonal angles are the angles formed between two diagonal line segments in a polygon. In a square, the diagonal angles measure 90 degrees. In other polygons, the diagonal angles depend on the number of sides. For example, a hexagon has diagonal angles of 120 degrees.


A diagonal bisector divides a diagonal line segment into two equal parts. It’s basically the midpoint of the diagonal. Diagonal bisectors are significant because they allow you to prove congruent triangles within polygons.


The length of a diagonal depends on the polygon. For example, in a square with side length ‘s’, the diagonal is s√2. In an equilateral triangle with side ‘s’, the diagonal is s√3/2. The diagonal lengths follow the Pythagorean theorem based on the polygon’s dimensions.

With all these diagonals, line segments, angles and lengths, you’ll be analyzing polygons like a pro. Diagonals are more than just slanted lines – they’re geometric necessities that help prove shapes’ properties and open the door to new discoveries. Keep exploring!

Using Diagonals to Find Area and Perimeter

Diagonals are useful for calculating the area and perimeter of shapes like squares, rectangles, and triangles. Let’s look at how to use them:

Finding the Area of a Square

The area of a square is equal to the length of one side squared. Since all sides of a square are the same length, you only need to measure one side.

Let’s say one side of your square measures 6 inches.

6 inches x 6 inches = 36 square inches

So the area of your square is 36 square inches. Easy as pie!

Finding the Perimeter of a Rectangle

To calculate the perimeter of a rectangle, you need to measure two sides: the length and the width. Add the length of all four sides to get the total perimeter.

For example, if your rectangle is 10 inches long and 5 inches wide:

10 inches (length)

  • 5 inches (width)
  • 10 inches (length)
  • 5 inches (width)

= 30 inches (perimeter)

The perimeter of your rectangle is 30 inches.

Finding the Area of a Triangle

To get the area of a triangle, you need to know the length of its base and height. The base is the bottom side of the triangle, and the height is a line drawn from the top point of the triangle straight down to the base.

Multiply the base by the height, then divide by two:

Area = (Base x Height) / 2

For instance, if your triangle has a base of 8 inches and a height of 6 inches:

(8 in. x 6 in.) / 2 = 24 in. / 2 = 12 in.

The area of your triangle is 12 square inches.

Interesting Properties of Diagonals

Diagonals have some interesting properties worth knowing about.

They bisect shapes

The diagonals of a square, rectangle, rhombus, and parallelogram bisect each other. That means they cut each other into two equal halves. For example, the diagonals of a square intersect at right angles, dividing the square into four smaller but congruent squares.

They are perpendicular bisectors

In a rectangle, parallelogram or rhombus, the diagonals bisect each other at right angles. Each diagonal cuts the other one in half at a 90° angle.

They are the same length in some quadrilaterals

In squares, the diagonals are the same length. In a rhombus, the diagonals are also the same length. This is not true for rectangles or parallelograms, however. Their diagonals can be different lengths.

They determine whether a shape is a parallelogram

If a quadrilateral has two pairs of parallel sides and diagonals that bisect each other, it must be a parallelogram. The parallel sides and intersecting diagonals are what characterize this four-sided figure.

They help calculate area

For a parallelogram, you can calculate its area by multiplying the length of one of its diagonals by the length of one of its sides. For a rhombus, multiply one-half the length of a diagonal by the length of one of its sides. The diagonals provide a convenient way to determine the area of these shapes without having to calculate all the side lengths.

Knowing these properties of diagonals will give you a better understanding of quadrilaterals and their characteristics. Diagonals may seem like a simple concept, but they have some interesting mathematical traits worth exploring.

Real World Applications of Diagonals

Diagonals have many practical applications in the real world. They are used in structures, mechanisms, and designs all around us.

Structural Support

Diagonals provide stability and support in building construction. Roof trusses, bridges, and scaffolding all use diagonal cross bracing to evenly distribute weight and prevent collapse. The diagonal members create triangles which are one of the strongest geometric shapes.

Movement and Motion

Diagonals enable smooth and controlled movement in machines and tools. Pantographs, scissors, pliers and other instruments all pivot and cut using diagonal linkages. Diagonals are also essential for the suspension systems of vehicles like cars, trains, and bicycles.


Diagonals provide the shortest path between two points, so they are used when directness and efficiency are priorities. Staircases are built with diagonal steps to quickly ascend or descend. Packaging often uses diagonal folds and seams to enclose the maximum volume using the least amount of material.


Diagonals are visually interesting and help create dramatic or dynamic effects in art, graphic design, and architecture. Paintings may use diagonal composition to create a sense of movement or tension. Buildings incorporate diagonal elements like slanted walls or asymmetric shapes for visual impact. Product designs use diagonal lines to appear more sleek, modern or aerodynamic.

In many areas of life, diagonals play an important functional and esthetic role. Their strength, efficiency, and visual appeal have made them a key element in engineering, construction, and design. Diagonals truly shape the world around us, from the buildings we inhabit to the tools and machines we use every day.


So there you have it, a straightforward explanation of what a diagonal is and how to spot one in all its slanty glory. Diagonals are more than just angled lines in geometry – they surround us in the buildings we inhabit, the sports we play, and even the keyboards we type on. Keep your eyes open and you’ll start noticing diagonals everywhere in the world around you. Whether in art, architecture, or nature itself, the diagonal adds visual interest, dynamism and a dash of fun to our lives. Pretty amazing what a little slanted line can do. Now get out there and appreciate the diagonal!

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