Understanding Line Types 43
used to represent partially drawn views and partial sections of symmetrical parts. Symmetrical-view vis-
ible and hidden lines may be extended past the symmetrical line if this improves clarity.
Extension Lines.
Extension lines are used in dimensioning to show the size of an object. Extension lines are thin, dark,
solid lines that extend from an object at the exact locations between which dimensions are to be placed.
A space of approximately 1/16 inch is usually allowed between the object and the beginning of the ex-
tension line (Figure 3.6). Plan to avoid crossing other extension lines and/or dimension lines.
Extension lines are fine lines that relate the dimension lines to their features. They do not touch the
features; instead they start about 1/6 inch (4.23 mm) from the feature and extend about 1/8 inch (3.175
mm) beyond the arrowheads of the dimension line.
Extension lines show the extent of a dimension, and dimension lines show the length of the dimen-
sion and terminate at the related extension lines with slashes, arrowheads, or dots. The dimension num-
ber in feet and inches is placed above and near the center of the solid dimension line.
Figure 3.4B Sectional lines add emphasis to the walls in this illustration.