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Parallel Sides: Definition & Concept

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  • 0:05 Definition of Parallel
  • 0:34 Properties of Parallel Sides
  • 1:07 The Parallel Symbol
  • 1:45 Shapes with Parallel Sides
  • 2:35 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this lesson, you will learn the special properties of parallel sides. Only certain shapes have the distinguished honor of having parallel sides. Learn what these are and also learn how to mathematically mark two sides that are parallel.

Definition of Parallel

Two sides or lines are parallel if they are lines that are always the same distance from each other and will never intersect or touch.

Parallel lines.
parallel sides

The two lines above are parallel. Can you see how they are always the same distance from each other no matter where you go on either line? If you and a buddy each walked on a line, the two of you would never cross paths, and if you kept pace with each other, you would always be the same distance apart. That is what it means to be parallel.

Properties of Parallel Sides

When a shape has a pair of parallel sides, it gives the shape certain unique properties.

A trapezoid with one pair of parallel sides.
parallel sides

Look at this trapezoid. Usually when a shape has a pair of parallel sides, they make up the bases of the shape. This trapezoid has two bases with each base being one of the two parallel sides.

Also, when a shape has a pair of parallel sides, the shape's height will be the same regardless of how long you stretch it out. Take this trapezoid, for instance. If I were to stretch the shape out, it would still have the same height.

Trapezoid has same height even when stretched because of the parallel sides.
parallel sides

The Parallel Symbol

There are two ways to mathematically notate that two sides are parallel to each other. One way is by writing it with a math symbol. Another way is to note it directly on the shape using matching arrow marks on the parallel sides.

The parallel symbol.
parallel sides

If a shape had more than one pair of parallel sides, then you would mark the difference by adding one more arrow mark to the second pair of parallel sides.

Marking two pairs of parallel sides.
parallel sides

Note how this rectangle has two pairs of parallel sides and how I've marked each pair differently. The second pair has two matching arrow marks instead of just one. This lets me know that while each pair is parallel, the two pairs are not parallel to each other.

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