UNIT
Alphabet of Lines
The fact that drawings are used in construction for the communication of information was discussed earlier. The drawings, then, serve as a language for the
construction industry. The basis for any language is its alphabet. The English
language uses an alphabet made up of twenty-six letters. Construction drawings use an alphabet of lines, Figure 4–1.
The weight or thickness of lines is sometimes varied to show their relative
importance. For example, in Figure 4–2 notice that the basic outline of the
building is heavier than the windows and doors. This difference in line weight
sometimes helps distinguish the basic shape of an object from surface details.
Objectives
After completing this unit,
you will be able to identify
and understand the meaning
of the listed lines:
Object lines
Dashed lines (hidden and
phantom)
Object Lines
Object lines are used to show the shape of an object. All visible edges are represented by object lines. All the lines in Figure 4–2 are object lines. Drawings
usually include many solid lines that are not object lines, however. Some of
these other solid lines are discussed here. Others are discussed later.
Extension lines and
dimension lines
Centerlines
Leaders
Cutting-plane lines
(a)
(b)
(c)
(d)
(e)
(f)
HEAVY SOLID (FOR OBJECT LINES)
LIGHT SOLID (FOR DIMENSIONS, LEADER, AND
EXTENSION LINES)
DASHED (FOR HIDDEN AND PHANTON LINES)
DOT DASH (FOR CENTERLINES)
DOT DOT DASH (FOR CUTTING PLANES)
BREAK LINES
Figure 4–1. Alphabet of lines
Figure 4–2. Elevation outlined. Courtesy of Robert C. Kurzon.
Alphabet of Lines 19
Figure 4–3. The dashed lines on this foundation plan indicate the footing.
Dashed Lines
Dashed lines have more than one purpose in construction
drawings. One type of dashed line, the hidden line, is used
to show the edges of objects that would not otherwise
be visible in the view shown. Hidden lines are drawn as
a series of evenly sized short dashes, Figure 4–3. If a
construction drawing were to include hidden lines for all
concealed edges, the drawing would be cluttered and
hard to read. Therefore, only the most important features
are shown by hidden lines.
Another type of dashed line is used to show
important overhead construction, Figure 4–4. These
dashed lines are called phantom lines. The objects
they show are not hidden in the view—they are simply not in the view. For example, the most practical
way to show exposed beams on a living room ceiling
may be to show them on the floor plan with phantom
lines. Phantom lines are also used to show alternate
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Figure 4–4. The dashed lines on this floor plan indicate the
edge of the roof overhang.
positions of objects, Figure 4–5. To avoid confusion,
the dashed lines may be made up of different weights
and different length dashes, depending on the purpose,
Figure 4–6.
Extension Lines and
Dimension Lines
Dimension lines are solid lines of the same weight as
extension lines. A dimension line is drawn from one extension line to the next. The dimension (distance between the
extension lines) is lettered above the dimension line. On
construction drawings, dimensions are expressed in feet
and inches. The ends of dimension lines are drawn in one
of three ways, as shown in Figure 4–8.
Extension lines are thin, solid lines that project from
an object to show the extent or limits of a dimension.
Extension lines do not quite touch the object they indicate, Figure 4–7.
Figure 4–5. The dashed lines here are phantom lines to show
alternate positions of the double-acting door and the door of the
dishwasher.
Figure 4–7. Dimension and extension lines.
Figure 4–6. Different types of dashed lines are used to show different features.
Alphabet of Lines 21
Figure 4–8. Dimension line ends.
Figure 4–10. When centerlines show the center of a round
object, the short dashes of two centerlines cross.
Figure 4–9. This centerline indicates that the column is
symmetrical, or the same, on both sides of the centerline.
Dimensions that can be added together to come up
with one overall dimension are called chain dimensions.
The dimension lines for chain dimensions are kept in line
as much as possible. This makes it easier to find the dimensions that must be added to find the overall dimension.
Centerlines
Centerlines are made up of long and short dashes.
They are used to show the centers of round or cylindrical objects. Centerlines are also used to indicate that
an object is symmetrical, or the same on both sides of
the center, Figure 4–9. To show the center of a round
object, two centerlines are used so that the short dashes
cross in the center, Figure 4–10.
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Figure 4–11. Method of showing the radius of an arc.
To lay out an arc or part of a circle, the radius must
be known. The radius of an arc is the distance from the
center to the edge of the arc. On construction drawings,
the center of an arc is shown by crossing centerlines. The
radius is dimensioned on a thin line from the center to the
edge of the arc, Figure 4–11.
Rather than clutter the drawing with unnecessary
lines, only the short, crossing dashes of the centerlines
are shown. If the centerlines are needed to dimension
the location of the center, only the needed centerlines
are extended.
Figure 4–12. Leaders used for dimensioning.
Leaders
Some construction details are too small to allow enough
room for clear dimensioning by the methods described
earlier. To overcome this problem, the dimension is
shown in a clear area of the drawing. A thin line
called a leader shows where the dimension belongs,
Figure 4–12.
Cutting-plane Lines
It was established earlier that section views are
needed to show interior detail. In order to show where
the imaginary cut was made, a cutting-plane line is
drawn on the view through which the cut was made,
Figure 4–13. A cutting-plane line is usually a heavy
line with long dashes and pairs of short dashes. Some
Figure 4–13. A cutting-plane line indicates where the imaginary
cut is made and how it is viewed.
drafters, however, use a solid, heavy line. In either
case, cutting-plane lines always have some identification at their ends and arrowheads to indicate the
direction from which the section is viewed. Cuttingplane-line identification symbols are discussed in the
next unit.
Some section views may not be referenced by a
cutting-plane line on any other view. These are typical
sections that would be the same if drawn from an imaginary cut in any part of the building, Figure 4–14.
Alphabet of Lines 23
Figure 4–14. Building section.
CHECK YOUR PROGRESS
Can you perform these tasks?
Identify and explain the use of object lines.
Identify and explain the use of hidden lines.
Identify and explain the use of phantom lines.
Identify and explain the use of dimension and extension lines.
Identify and explain the use of centerlines.
Identify and explain the use of leaders.
Identify and explain the use of cutting-plane lines.
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ASSIGNMENT
Refer to the drawings of the Two-Unit Apartment in the
packet. For each of the lines numbered A5.1 through
A5.10, identify the kind of line and briefly describe its
purpose on these drawings. The broad arrows with A5
numbers are for use in this assignment.
Example: A5.E, object line, shows end of building.