1.2: Line types

Learning Objectives

By the end of this section, learners will be able to

1. Identify common line types used in technical drawings.
2. Explain the purpose and application of various line types in technical drawings.
3. Interpret technical drawings accurately by understanding the meaning of each line type.

The language of technical drawing

Technical drawing is a form of communication. Like a language, it has its grammar and rules. Being familiar with these rules is essential for creating and interpreting a drawing correctly. Unlike other languages, technical drawings are universally understood— for example, a drawing created in the U.S. can be easily shared with professionals in different countries without requiring a “translation."

The initial lectures of this course introduce foundational concepts in technical drawing and drawing standards— including line types, measurement systems, and scale— which will support learners' understanding of the subsequent modules.

Line types:

Technical drawings consist of different line types, each having a precise meaning. For example, visible lines, also called object lines, represent visible edges of an object, while hidden lines represent edges that are hidden from that view. Line types are characterized by line weight (thin or thick lines) and style, such as continuous or dashed lines. To correctly create and interpret a technical drawing, it is essential to know the meaning of the different line types composing the drawing.

This lecture introduces 10 different line types, describing their characteristics of line weight and style, and their meaning. At this stage of the course, some of this information may seem abstract. As learners progress through this course, they will become familiar with different line types. The material presented in this lecture will serve as a foundation for upcoming modules on orthographic projections, multiview drawings, and section views. It will be referenced as new line types are introduced and applied.

The correct use of line types not only ensures that drawings are clear and easy to read, but it also allows for a correct interpretation of the drawing. The following examples demonstrate the importance of using different line types in technical drawings.

Figure \(\PageIndex{1}\): Sketch of a box using a single line type—a visible line (thick and continuous).

The sketch above represents a box. The drawing was created using only one line type: a thick, continuous line. This line type is called a visible or object line, and it is used to depict visible edges. However, if the box is a solid object, not all edges should be visible from this view. To interpret the drawing correctly, visible and hidden edges should be distinguished using two different line types. Although this example clearly depicts a box, it has two different interpretations, depending on which edges are considered to be hidden.

Figure \(\PageIndex{2}\): Interpretation 1: The front, right-side, and top surfaces of the object are visible.

In this first interpretation, the line type of the three edges highlighted in red in Figure 1.2.2(a) is changed to a hidden line in Figure 1.2.2(b)—a thin, dashed line representing edges of the object that are not visible in that view. In this interpretation, the box is rotated so that the front, right side, and top surfaces of the box are visible, as shown in Figures 1.2.2(c) and (d).

Figure \(\PageIndex{3}\): Interpretation 2: The front, left-side, and bottom surfaces of the object are visible.

The images above illustrate a second interpretation. As in the previous example, the line type of the three edges highlighted in red in Figure 1.2.3(a) is changed from a visible line to a hidden line in Figure 1.2.3(b). In this case, the front, left-side, and bottom surfaces are visible. It may take a moment to adjust to the new orientation.

Figure \(\PageIndex{4}\): Sketch of two overlapping boxes using a single line type (visible line).

The image above illustrates a more complex object to highlight further the importance of distinguishing between different line types in technical drawings.

Figure \(\PageIndex{5}\): Interpretation 1: Large box with a smaller one carved into its front face.

By changing the line type of the edges highlighted in red in Figure 1.2.5(a) from visible to hidden lines (Figure 1.2.5(b)), the sketch now represents a large box with a smaller one carved into its front face.

Figure \(\PageIndex{6}\): Interpretation 2: Large box with smaller box protruding from the front face.

Similarly, changing the red-highlighted edges in Figure 1.2.6(a) from visible to hidden lines—see Figure 1.2.6(b)—transforms the interpretation into a larger box with a smaller box protruding from the front.

Note

Please note that these sketches are cabinet projections. While hidden lines are typically omitted in cabinet projections, they are included in these examples to help visualize and understand hidden edges. Cabinet projections are covered in Module C, section 3.4.

In addition to visible and hidden lines, technical drawings include many other line types, such as center lines, cutting plane lines, section lines, dimension lines, and extension lines. Some of these lines represent actual features of the object (e.g., visible and hidden lines), while others provide additional information, such as information about geometry (center lines), dimensions (dimension lines), surface differentiation (section lines), or the extent of movement of a feature (phantom lines).

List of line types

Below is a list of 10 different line types, accompanied by illustrations and brief descriptions.

Figure \(\PageIndex{7}\): Visible line (a) and hidden line (b).

Visible lines (a), also called object lines, are thick (0.7mm) and continuous lines and represent the visible edges of an object. Hidden lines (b) are thin (0.3mm) and dashed lines that indicate edges that are hidden in the current view.

Figure \(\PageIndex{8}\): Center line (c), dimension and extension line (d).

Center Lines (c) are thin, dashed lines used to indicate the central axes of circular features or geometries. Unlike hidden lines, center lines consist of alternating long and short dashes and extend beyond the edges of the object. Dimension lines are thin, continuous lines with arrows or ticks at the ends that include a numerical value indicating the length (dimension) of a feature. Extension lines extend from the object's edge or a central line to the dimension line, helping to define the limits of the measured part.

Figure \(\PageIndex{9}\): Cutting plane line (e) and section line (f).

Cutting plane lines (e) are used to describe where a section is cut through an object and the direction of sight. They are thick, dashed lines with arrows on the ends showing the line of sight. Section lines (f) are thin, continuous diagonal lines used to identify the surfaces of an object that lie on the cutting plane.

Figure \(\PageIndex{10}\): Long break line (g), short break line (h), and phantom line (i).

Long break lines (g) are used to shorten the representation of a continuous object or feature, similar to the use of ellipses (i.e. ...) in writing. Short break lines (h) are thick, irregular lines used in broken sections to show only a portion of an object in a sectional view. Phantom lines (i) are thin, dashed lines that indicate the range or extent of motion of a moving feature.