Copyright Goodheart-Willcox Co., Inc. Unit 2 Line Conventions and Lettering 21 Another version represented in the standard is a se- ries of medium-size dashes, about twice as long as hidden-line dashes. The current standard also allows for two “elbows” with identifying labels and arrows. In any case, the lines should be the same thickness as visible lines. Dimensioning Lines While the visible, hidden, and center lines are used to create the shape description of an object, the size description of an object is indicated in annotations known as dimensions. Dimensions are composed of a variety of lines, all drawn without dashes and all with a thin line weight. Dimensioning is discussed in Unit 9. The lines that extend the edges of the object out away from the view are called extension lines, Figure 2‑6. Some books, references, and CAD systems refer to extension lines as “witness” lines. The purpose of the extension lines is to keep the dimensional anno- tation away from the shape description. Extension lines begin about 1/16″ away from the object’s visi- ble corners and extend about 1/8″ beyond the arrows of the dimension line. As shown, extension lines can cross each other without issue. Between the extension lines are dimension lines, which indicate the extent and direction of the dimensions, Figure 2‑6. In most engineering draw- ings, the dimension line is broken in the middle for the dimensional value. Dimension lines are usually terminated by arrowheads against extension lines. While extension lines can cross each other, the drafter tries to avoid dimension lines crossing each other. Dimensioning methods are becoming more diverse in industrial applications, including arrow- less coordinate dimensioning and geometric dimen- sioning and tolerancing. These topics are discussed in greater detail in later units. In dimensioning, leader lines, commonly known as leaders, are used to point to a feature, such as a hole, or a drawing area to which a local note applies, Figure 2‑6. Leaders are also used in conjunction with dimension lines if there is insufficient room Goodheart-Willcox Publisher Figure 2‑4. Center lines are thin lines with alternating long and short dashes. They designate centers of holes, arcs, and other symmetrical objects. Center lines Hidden lines