Head First 2D Geometry

6 Chapter 1

right angles

Right angles aren’t always marked with numbers

A right angle is an internal angle (the smallest angle) between

two lines equal to 90º. Instead of drawing a curve, like we do

for most angles, we mark it with a square corner.

A right angle is a quarter turn—like the angle between the

hands on the clock at 3 o’clock or the amount you have to

turn a skateboard to get into and out of a boardslide without

landing on your face.

Lines that meet at a right angle are perpendicular

Whether they actually cross each other or only meet

at a point, two lines that form a right angle are called

perpendicular lines. We could also say that one of

the lines is perpendicular to the other.

So it turns out that we do have five

angles on the sketch after all.

Although they don’t say “90º” on them, the little square

angle marks tell us that some of the angles are right angles.

But what about all those other angles that aren’t marked

on the sketch?

This means

the same as this.

Perpendicular line

segments that meet

Perpendicular

lines segments

that cross

It is still possible to land on your face

even if you make the 90º turn.

Slide rail

90º turn

90º turn

Skateboard

90º

Line segments are parts of lines

In geometry jargon, a line goes on

forever in both directions. A line segment is

just a bit of a line with a start and an end.