Example 2: 15:20 = 3:4
Parallel--Maintaining an equal distance at all
points on two or more lines or plants.
Example 3:
Perpendicular-- Right angles (90) to a line or
plane.
As shown in the examples, a proportion is nothing
Plane-- A two--dimensional geometric figure.
more than an equation of common fractions. The value
of proportion is that if any three of the terms are given,
Point--The intersection of two lines. A point has
the fourth or unknown term can be found. This is done
no dimensions.
by solving a simple problem of common fractions.
Polygon-- Any closed figure having three or more
sides.
GEOMETRIC CONSTRUCTION
Radius--The distance from the center of a circle to
Patterns are geometric shapes that conform to a
the circumferece of that circle (one-half of the
draftsman's plan (blueprint) and contain allowances for
diameter).
draft, shrinkage, and machine finish. To construct a
T a n g e n t --A curved or straight lint that touches but
pattern, you must solve graphics problems of geometric
construction. Graphics problems can be solved by trial
does not cross another curved or straight line at a point
and error or by measuring with a scale. However,
other than its ends.
neither method is accurate; therefore, they cannot be
Vertex--The point of intersection of two lines
used in patternmaking. Because of its accuracy, only
forming an angle.
the geometric method of construction should be used in
patternmaking. No other method is acceptable.
BISECTING A LINE
NOTE: You will need specific tools to USC during
As you read this section, refer to figure 14-2. You
geometric construction. Sonic of these tools include a
arc going to learn how to bisect a line by using the
pencil divider, a 12-inch scale, a flexible straightedge, a
following steps:
30- to 60-degree triangle, and a T-square.
1. USC your dividers and adjust them with a spread
As you read about geometric construction, the
that is visually greater than one-half the length
discussion will contain some geometric terms that you
of the line.
should know. These terms arc defined as follows:
2. Insert one point of the dividers at one end of the
Angle-- A figure formed by two lines converging on
line (point A, view A) and draw an arc. Be
a common point .
careful not to change the adjustment on the
Apex-- The highest point of a triangle.
dividers.
Arc--A portion of a circle or curve.
3. Insert one point at the other end of the line (point
B, view B) and draw an arc from that end
Bisect-- To divide into two equal parts.
intersecting the first arc.
Circumference-- The distance around a circle,
4. Draw a straight line connecting the two
ellipse, or closed curve.
intersection points of the arcs to bisect the line
Curve--Any geometric line or shape that is not
(view C).
straight, contains no angles, and does not form a closed
figure.
Diameter--The distance from a point on the
circumference of a circle, through the center, to the
opposite point on a circle.
Hypotenuse--The side of a right triangle that is
opposite the right angle.
Intersect--To meet and cross at a point or series of
points. Two lines intersect at one point. Two plants
Figure 14-2.--Bisecting a line.
intersect at a series of adjacent points (which is the
definition of a line).
14-7
