A square line from a point
Show on a big screenA shelf must stand at 90° to the wall.
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- A line, and a point Q off the line.
- An arc from Q cuts the line at A and B.
- Equal arcs from A and B cross at Q and below.
- Rule through both crossings. It meets the line at 90°.
The rule
A perpendicular meets a line at 90°.
To draw a perpendicular, you need a segment on the line. Its perpendicular bisector must go through your point. On the line, make the point the middle of the segment. Off the line, pick ends that are equally far from it.
Point on the line: mark A and B, equal steps away. Strike equal arcs from A and B. Rule through both crossings.
Point off the line: strike an arc from the point. It must cut the line twice. Those two cuts are A and B. Bisect AB as before.
Example: a point is 26 mm above a line. A span of 22 mm falls short. A span over 26 mm cuts it twice.
Watch out!
If the arcs do not cross, the span is too short. Set the span to more than half of AB.
Try the lab to finish this lesson.