Drawing symbols and legends are the conventions a drawing uses to say things without words: line types, symbols, hatches and abbreviations. The legend is the key that defines them for a given set. Two sets can use the same symbol for different things, so the legend of each set is what counts.
At a glance
| Line types | Continuous, dashed (hidden), chain (centre and grid lines) |
| Symbols | North point, section and detail marks, door swings, levels, grid bubbles |
| Hatches | Materials in section: concrete, masonry, earth, insulation |
| Abbreviations | Short text such as El., Sta., c/c, TYP, NTS |
| Where defined | The legend sheet, the general notes, or the title sheet |
The main conventions
| Convention | Usually means |
|---|---|
| Thick continuous line | Cut edge of what is built |
| Thin continuous line | Edge seen beyond, dimension or leader |
| Dashed line | Hidden, below or behind; sometimes existing to be removed |
| Chain line (long and short dashes) | Centre line or grid line |
| Section mark (arrow with a reference) | Where a section is cut, and the sheet it is drawn on |
| Detail callout (circle with a reference) | A part drawn larger elsewhere |
| Level symbol | A height above the project or survey datum |
| TYP | Typical: applies wherever nothing else is shown |
| NTS | Not to scale |
How they are used on projects
Each office and each discipline has its own standard symbols, and many countries publish national drawing conventions. On a project, the information standard or a shared template should fix one legend for the set, so that all teams' sheets read the same way. Model-based drawings produce most symbols automatically from the model; hand-drawn symbols and text are where errors and inconsistencies tend to creep in.
Civil and highway drawings add their own abbreviations for alignments and levels, such as station (Sta.), point of intersection (P.I.), vertical curve (V.C.) and crown of roadway (C/R).
Variations
- Architectural, structural and services drawings each have their own symbol sets.
- Imperial and metric sets differ in how scales and levels are written.
- Historic sets often define abbreviations nowhere: they were common knowledge in the office that drew them.
Common mistakes
- Assuming a symbol means what it meant on the last project.
- Reading an abbreviation by guesswork instead of finding the legend, or asking.
- Measuring a "not to scale" detail.
- Mixed legends within one set, because teams used their own templates.
Where to read more
National drawing practice standards (for example the ISO 128 series on technical drawing conventions and your country's CAD standards); the project's information standard; and the legend and general notes of the set itself.