Line types, weights and lettering
Every line on an engineering drawing means something specific. Learn the standard line types, their weights, and lettering rules that keep drawings unambiguous.
In this lesson you will
- Identify standard line types and their uses
- Apply line weight hierarchy
- Use centre lines correctly
- Letter drawings legibly to standard
Line types
| Line | Look | Use |
|---|---|---|
| Visible (outline) | Thick continuous | Visible edges and outlines |
| Hidden | Dashed | Edges hidden from view |
| Centre | Thin chain (long–short) | Axes, centres of holes, symmetry |
| Dimension / extension / leader | Thin continuous | Dimensioning and notes |
| Cutting plane | Chain, thick at ends and changes of direction | Where a section is taken |
| Phantom | Thin chain with two short dashes | Alternative positions, adjacent parts |
| Break | Freehand or zigzag | Shortened views |
Weights
Two or three weights are typical, e.g. 0.7 mm thick and 0.35 mm thin (ISO 128 uses a ratio of about 2:1). Outlines stand out; everything that explains the outline stays thin. In CAD, set weights on layers and check with a print.
Centre lines
Every hole and cylindrical feature gets centre lines crossing at the centre (on the dashes, not the gaps), extending a little beyond the feature. Symmetrical parts show the axis of symmetry as a centre line.
Lettering
Use a plain, standard font (ISO 3098 describes lettering). Typical heights: 2.5–3.5 mm for notes and dimensions, 5–7 mm for titles and view labels. Upper case is traditional in many standards. Keep notes short and consistent.
Common mistakes
Using one line weight for everything.
Keep outlines thick and all annotation thin so the shape reads instantly.
Hidden lines everywhere.
Use a section view instead when internal detail becomes cluttered.
Decorative or condensed fonts.
Use a plain standard font at a legible height.
Exercise
A line-type sheet
- Draw a simple bracket with a hole and a hidden slot.
- Apply correct visible, hidden and centre lines.
- Add a cutting plane and label it A–A.
- Print and check weights are clearly distinguishable.
You should end up with: A drawing with correct line conventions.
Checklist
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Lesson quiz
Match the line to its use.
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