Side — Definition, Formula & Examples
A side is one of the straight line segments that forms the boundary of a polygon. For example, a triangle has 3 sides and a rectangle has 4 sides.
In a polygon, a side is a line segment connecting two consecutive vertices. The number of sides determines the type of polygon and equals the number of its vertices and interior angles.
How It Works
To find how many sides a shape has, count each straight edge along the outside of the polygon. A polygon is named by its number of sides: 3 sides make a triangle, 4 sides make a quadrilateral, 5 sides make a pentagon, and so on. Two sides meet at each vertex (corner) of the shape. When all sides have the same length, the polygon is called equilateral.
Worked Example
Problem: A regular hexagon has a perimeter of 42 cm. How long is each side?
Count the sides: A hexagon has 6 sides. Since it is regular, all 6 sides are equal in length.
Divide the perimeter by the number of sides: The perimeter is the total length of all sides added together.
Answer: Each side of the hexagon is 7 cm long.
Why It Matters
Knowing what a side is lets you calculate perimeter, identify polygon types, and work with area formulas. Nearly every geometry problem about shapes starts with understanding their sides.
Common Mistakes
Mistake: Confusing sides with diagonals.
Correction: A side connects two consecutive (neighboring) vertices along the boundary. A diagonal connects two non-consecutive vertices and cuts through the interior of the shape.