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Right Angle


  • Why is it called a right angle? Are there left angles or wrong angles?
  • Making a right angle by paperfolding.

Informally, a right angle is the angle at the corner of a (rectangular) piece of paper. Other examples include the angle at the base of a capital L or the top of a capital T. The measure of a right angle, in degrees, is 90°.


Informal: an angle that matches the corner of a (rectangular) piece of paper, or the angle at the base of a capital L or the top of a capital T.

These are all examples of right angles: Image:RightAngles.png


These are not right angles: Image:RightAngleNot.png The two at the left are both obtuse ("wider" than a right angle) and the two at the right are both acute ("sharper" than a right angle).


Paperfolding to construct a right angle

You can construct a right angle just by folding paper, even one that has been ripped so that there are no straight edges!

In any direction, fold the piece of paper. For example:

The result will look like this:

Then make a second fold, being careful that the original crease lies on top of itself.

The result will look like this:

When you unfold the piece of paper, the two creases will be perpendicular to each other. The four angles are all right angles.

Why does this work? The first fold creates a straight-line crease. The second fold folds one part of that line onto the other. If an angle is folded through its vertex so that one side of the angle lies directly on the other, the fold bisects the angle. The straight line of the original crease forms a straight angle (180°) around any point on it. Folding through that point bisects the angle, so the resulting angles are each 90°, and so they are right angles.

AnchorRelated mathematical ideas

  • In informal speech (not mathematics), right-angle corners are sometimes called square corners even though no complete square may exist (as in the examples above).
  • Four right angles fit around a point; for example, when we draw two diameters to cut a circle into fourths, the four angles that fit around the center of the circle are right angles.
  • The way we define degrees, a "full turn" around a point is 360°, so the measure of each right angle is one fourth of that: 90°.
  • Two lines that are at right angles to each other are said to be perpendicular to each other.
  • The word-parts rect- and -angle in"rectangle" literally mean "right angle."

What's in a word?

The gh in "right" was once pronounced; it descended from the c in Latin "rect-." In English, words with "rect-" in them, like rectangle, often have meanings related to "right."

AnchorRight has many meanings

One of its meanings is 'correct,' the opposite of 'wrong.' (When we get something wrong, we try to rectify our error.) Another meaning is the opposite of 'left.' Still another meaning is 'straight' or 'directly,' as in "After school, please come right home."

Why all those meanings? What do they all have in common? Like most words, right has great-…-great grandparents. The earliest ancestor we know might have sounded something like reg, and many of right's cousins are spelled like reg or rect.

The original meaning was something like 'to move in a straight line.' When we regulate something, we are keeping it on track, keeping it from changing, keeping control. 'Following the straight path' means doing the 'right' thing. The person who gets to make all the rules and regulations that keep people in line is the king. The Latin word for king is 'rex,' related to reg and rect. Tyrannosaurus Rex was 'king' of the dinosaurs.

When the g in regular fades into a vowel, we get its cousin, ruler.

The g fades into a vowel in many English words. In words like light and though and many other gh words, the g stays in the spelling, but just disappears in pronunciation. In some words, like rule and royal, it even drops out of the spelling. Royal is regal (from 'rex' the king) with a vowel-like g. When we draw an answer out of someone, we are dragging it, with a vowel-like g! And look up the meaning of dray. That also comes from draging. Guardian and warden and even yard are related the same way.

When the g in regular is silent (as it is in right, and many other words), we get ruler. A ruler helps you draw a straight line.

The royal, regal king was the richest (yes, that's related, too), most powerful person, who gave people their rights, or took them away! When we name children Richard or Regina or, of course, Rex—or Henry or Roy with yet another silent g—we are comparing them to kings and queens!

If right means something like 'straight,' why do we say right turn? Rex, the royal, ruling king, is strong, and so right suggests strength. The strong hand, for most people, is their right hand, so a turn to the right is a turn toward that strong right hand.

AnchorWhere does a right angle get its name? Are there "left angles," too?

No, the right angle has nothing to do with right or left turns.

In this map, we say that Walnut Street makes a right angle with Main Street because it goes as 'straight' away from Main Street as possible. Corn Street veers off at a different angle.