Line types on technical drawings
Name the line marked in blue. Get 10 right to finish. Pick ASME (US) or ISO (Europe) in the box below.
What is the line marked in blue?
How to read it
Every line on a drawing has a pattern and a weight, and together they tell you what it means. US drawings follow ASME Y14.2 and name lines by what they do. ISO 128 names them by how they look: continuous, dashed, long-dashed dotted, and wide or narrow.
| Looks like | ASME name | ISO name | Used for |
|---|---|---|---|
| Visible (object) line | Continuous wide line | Edges and outlines you can see | |
| Hidden line | Dashed narrow line | Edges behind the surface you are looking at | |
| Center line | Long-dashed dotted narrow line | Axes of holes and round parts, symmetry | |
| Phantom line | Long-dashed double-dotted narrow line | Mating parts, other positions of a moving part | |
| Dimension, extension and leader lines | Continuous narrow line | Sizes and notes | |
| Cutting-plane line | Long-dashed dotted line, wide at the ends | Where a section view is cut, with arrows for the viewing direction |
The cutting-plane line differs most. ASME draws it thick, often in the phantom pattern. ISO draws it as a narrow chain line made wide at the ends and where it changes direction. Both add arrows for the viewing direction and a letter, such as A, that names the section.
Common mistakes
- Mixing up center lines and phantom lines. Count the short dashes: one for a center line, two for a phantom line.
- Reading a phantom outline as part of the part. It shows something else, such as the part it mates with.
- Taking a dimension from an extension line. The size is carried by the dimension line between the arrows.
Questions
- What is the difference between a hidden line and a center line?
- A hidden line is made of short dashes of the same length and shows an edge you cannot see. A center line alternates a long dash and a short dash and marks an axis, such as the middle of a hole.
- What is a phantom line used for?
- To show a part that is not the one being drawn, such as a mating part, or another position of a moving part. It is thin, with a long dash and two short dashes.
- Why are some lines thick and others thin?
- Thick lines show the part itself so it stands out. Thin lines carry information about the part: dimensions, center lines and hidden edges. ASME uses two widths, ISO uses line groups with wide and narrow lines.
- What happens when two lines fall on top of each other?
- Only one is drawn. A visible line wins over a hidden line, and a hidden line wins over a center line.
Explained in our own words and checked against several textbooks. For the full rules, see ASME Y14.2, ASME Y14.3, ISO 128, ISO 5456-2, ISO 7200 and ISO 2768-1.