A side is one of the straight line segments that form the outline of a polygon. Two sides meet at each corner, or vertex. A triangle has 3 sides and a square has 4, and adding the lengths of all the sides gives the perimeter.
Add the sides to get the perimeter. In a triangle, each side is named by the angle it faces; on a solid, the flat parts are faces and the lines are edges.
Formal definition and notation
In a polygon A1A2 … An, the sides are the n segments A1A2, A2A3, …, AnA1 joining consecutive vertices. Each vertex is the shared endpoint of exactly two sides, so a polygon has as many sides as vertices. The side length is the length of that segment.
A side is a segment; a vertex is a point where two sides meet. A square has 4 sides and 4 vertices.
Sidevs.Edge
Edge is the word for a segment on a solid figure, where two faces meet. A cube has 12 edges; a square face of it has 4 sides.
Quick check: 3 questions
Question 1 of 3
Each side of a regular octagon is 5 cm. What is its perimeter?
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Answer: B. An octagon has 8 sides: 8 × 5 = 40. Choice A uses 6 sides, C uses 5, and D adds 8 and 5.
Question 2 of 3
How many sides does a pentagon have?
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Answer: D. Penta- means 5, so a pentagon has 5 sides and 5 vertices. A quadrilateral has 4, a hexagon 6 and an octagon 8.
Question 3 of 3
In triangle ABC, which side is across from angle B?
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Answer: C. The side opposite an angle is the one that does not touch its vertex. AC does not touch B. Sides AB and BC both start at B, so they form angle B.
Side: frequently asked questions
How many sides does a circle have?
None. A side is a straight line segment, and a circle is a single curved line with no straight parts and no corners. So a circle is not a polygon, and instead of adding side lengths you measure the distance around it, called the circumference.
What is the difference between a side and an edge?
A side belongs to a flat shape: it is one of the segments of a polygon. An edge belongs to a solid: it is a segment where two faces meet. A cube has 6 square faces and 12 edges, and each square face has 4 sides. In everyday talk people also call a face a side, so read the problem carefully.
Line SegmentA line segment is a straight part of a line with two endpoints. It starts at one point, stops at another, and includes every point between them. A segment with endpoints A and B is written AB. Unlike a line, which goes on forever, a segment has a definite length that you can measure.Geometry · Grade 4, Grade 5, Grade 6PolygonA polygon is a flat, closed shape made of straight line segments, called sides, that meet only at their endpoints. Each corner is a vertex. Polygons are named by their number of sides: a triangle has 3, a quadrilateral 4, a pentagon 5, a hexagon 6 and an octagon 8.Geometry · Grade 2, Grade 3, Grade 5VertexA vertex is a corner point where two or more lines, rays, or sides meet. It is the point of an angle, a corner of a polygon such as a triangle, a corner of a solid, and the turning point of a parabola. The plural of vertex is vertices: a square has 4 vertices.Geometry · Grade 4, Grade 5, Grade 6PerimeterPerimeter is the total distance around a two-dimensional shape. To find it, add the lengths of all the sides. A rectangle with sides 5 and 3 has perimeter 5 + 3 + 5 + 3 = 16. Perimeter is a length, so it uses length units such as inches or meters, never square units.Geometry · Grade 3, Grade 4, Grade 5LengthLength is how long something is: the distance from one end of an object to the other. You measure it by counting equal units laid end to end with no gaps or overlaps, such as inches, feet, centimeters or meters. A new pencil is about 7 inches long.Measurement · Grade 1, Grade 2, Grade 3TriangleA triangle is a polygon with three sides and three angles. Its three corners are called vertices, and the angle measures in any triangle add to 180 degrees. Triangles are classified by their sides (equilateral, isosceles, scalene) and by their angles (acute, right, obtuse).Geometry · Grade 3, Grade 4, Grade 5NumberA number is an amount or value used to count, measure, order and compare. We write numbers with digits (7), words (seven) or other symbols, but the number itself is the amount: 7 dots, the word seven and the numeral 7 all show the same number. Numbers include whole numbers, fractions, decimals and negatives.Number System · Kindergarten, Grade 1, Grade 2CircleA circle is the set of all points in a flat plane that are the same distance from one fixed point, the center. That distance is the radius. A segment through the center from edge to edge is the diameter, twice the radius. The distance around is the circumference, 2π r, and the area inside is π r2.Geometry · Kindergarten, Grade 1, Grade 7Equilateral TriangleAn equilateral triangle is a triangle whose three sides all have the same length. Its three angles are also equal, so each one measures 60 degrees. With side length a, its perimeter is 3a and its area is √34a2. Every equilateral triangle is also isosceles and acute.Geometry · Grade 4, Grade 5, Grade 7QuadrilateralA quadrilateral is a polygon with four straight sides, four vertices and four angles. Its four interior angles always add to 360 degrees. Squares, rectangles, rhombuses, parallelograms, trapezoids and kites are special quadrilaterals, each named for extra properties of its sides or angles.Geometry · Grade 2, Grade 3, Grade 5Whole NumberA whole number is 0 or a positive counting number: 0, 1, 2, 3, and so on. Whole numbers do not include fractions, decimals, or negative numbers, so 12 is a whole number but 8.5, 34 and −3 are not. Every whole number is also an integer.Number System · Grade 3, Grade 4, Grade 5CircumferenceCircumference is the distance around a circle, found by multiplying the diameter by pi, or the radius by 2 pi: C = π d = 2π r. It is the circle’s perimeter, measured in units of length. A circle with diameter 10 has a circumference of about 31.4, because π ≈ 3.14.Geometry · Grade 6, Grade 7, Grade 8SquareA square is a quadrilateral with four equal sides and four right angles. It is a special rectangle and a special rhombus at the same time. For a square with side s, the perimeter is 4s, the area is s2, and each diagonal is s√2.Geometry · Kindergarten, Grade 2, Grade 3PointA point is an exact location in space. It has no length, no width and no thickness, so it has no size at all. You draw a point as a dot and name it with a capital letter, such as point A. On a coordinate plane, an ordered pair such as (3, 2) names a point.Geometry · Grade 4, Grade 5, Grade 6AngleAn angle is formed by two rays that share the same endpoint, called the vertex. It is measured in degrees by how far one ray is turned from the other. A right angle measures 90 degrees, a straight angle measures 180 degrees, and one full turn is 360 degrees.Geometry · Grade 4, Grade 5, Grade 6