Hamilton's Math To Build On - copyright 1993

Angles

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About Math To Build On || Contents || On to Crossing & Parallel Lines || Glossary
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Angles

Angle is an important term in the trades and has many definitions; however, three definitions cover most of the knowledge we need:

(1) The shape made by two straight lines meeting in a point.
(2) The space between those lines.

(4) The vertex is the point where the two straight lines come together to form the angle.

Angle Names

Names are given to angles with certain characteristics:

(5) A zero degree angle has no space between the two lines.
(6) An acute angle is any angle between 0° and 90° .
(7) A right angle is a 90° angle. The two lines that form a 90° angle are perpendicular to each other.
Notice that the angle symbol is in the shape of a box. That symbol indicates that you are
dealing with a right angle.
(8) An obtuse angle is an angle between 90° and 180° .

(9) A straight angle is an angle of 180° .
(10) A central angle is an angle of which the vertex is located in the center of a circle.
(11) Adjacent angles are angles that have the same vertex and one side in common.
(Adjacent means next to or near).
(12) Opposite angles are formed by two straight lines crossing and are always equal.

Seeing is Believing: Angle Construction Practice

The compass symbol lets you know there is related information in the section Seeing is Believing -- a very basic mini-drawing section, which is provided just in case you've forgotten, or never learned, some basic drawing rules.

90° angles

The angle most often used in the trades is the 90° angle.
It may help to understand why this is so.

When two straight lines cross, four angles are created.
Those four angles always add up to 360° .

The major lines of the compass are two straight
lines which cross each other at 90° angles.
Most plants and factories are said to be built along
the major lines of the compass.

When lines cross, the only time all four of the angles
created are equal to each other is when each angle is
a 90° angle. A 90° angle is called a right angle.

Seeing is Believing: Fractions of the Ruler

Much of our work is lined up to be level or
plumb.

If you drop a plumb bob line across a level line,
you again have two straight lines crossed at
90° angles.

Unless there are particular reasons for this not
to be so, everything on a construction job is set
up to be in line with a major compass line (axis)
and to be plumb or level. That means that almost
all things on a plant site are either parallel or
perpendicular to each other.

The easiest way to keep things that way is to make
a majority of the turns at 90° angles.

On to Crossing & Parallel Lines

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7 September 1995

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