Parallel and Intersecting Lines
Parallel lines
Learn what parallel lines are, and the right words for how lines meet.
Look at a figure of line segments and describe how each pair meets, using the right words: point, endpoint, midpoint, meet, intersect, and the angle. For example: *FG and FH meet at the endpoint F at an angle of 115.3°.*
Some pairs, like ST and UV, or OP and QR, look as if they would never meet, however long you made them.
Lines like that are all around: the two rails of a railway track, the two long edges of a ruler, the lines of shading in a drawing. They are called parallel lines.
The definition has two parts, and both matter. Parallel lines:
- lie on the same plane, and
- never meet, however far you extend them at both ends.
Why the same plane? A line drawn on your desk and a line on the blackboard may never meet, but they are not parallel: they are on different surfaces and never had a chance to meet.
Find parallel lines in your classroom: the opposite edges of the door, the bars of a window grille, the top and bottom edges of the blackboard.
In drawings, parallel lines are marked with arrow marks. A second set of parallel lines in the same figure gets two arrows, and so on. (Perpendicular lines get a small square.)
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