Read line types, drawing scales and the information in a title block.
Why it matters
Every line on a print means something specific: an edge you can see, an edge you cannot, an axis, a dimension, a cut. The scale tells you how the paper relates to the real object, and the title block tells you which part, which revision and whose responsibility it is. Building from an out-of-date revision or misreading a scale is one of the most expensive mistakes on a job.
The alphabet of lines
Standards (ASME Y14.2 in the US, ISO 128 internationally) define line types by thickness and pattern:
| Line | Looks like | Means |
|---|---|---|
| Visible (object) | thick, continuous | edges you can see |
| Hidden | medium, short dashes | edges behind the surface |
| Centre | thin, long-short-long | axes of holes, cylinders, symmetry |
| Dimension | thin, with arrowheads, number in the middle | a measured distance |
| Extension | thin, continuous, with a small gap from the part | extends a feature out to a dimension |
| Leader | thin, angled, with an arrow or dot | points a note or callout at a feature |
| Cutting plane | thick, long dashes with arrows, letters at the ends | where a section view is cut |
| Section (hatch) | thin, evenly spaced diagonal lines | solid material cut in a section view |
| Phantom | thin, long-short-short-long | alternate positions, adjacent parts |
| Break | freehand or zigzag | a long part shortened on the drawing |
The thick-thin contrast is deliberate: the eye picks out the part's outline first.
Section views
When interior features are complex, the drawing shows a section view: imagine cutting the part along the cutting-plane line, removing the piece toward the arrows, and looking at what is left. The view is labelled, for example SECTION A-A, and the cut surfaces are hatched.
Scale
Scale relates drawing size to real size.
- Metric and mechanical scales are ratios: 1:1 full size, 1:2 half size (1 mm on paper = 2 mm real), 2:1 twice size for small parts.
- Architectural scales (US building plans) are written as paper inches per real foot: 1/4" = 1'-0" means a quarter inch on the paper represents one foot (a ratio of 1:48). 1/8" = 1'-0" is 1:96.
- Engineering (civil) scales use one inch to a number of feet: 1" = 20' (1:240), 1" = 50', common on site plans.
Worked example: reading a scaled distance
A floor plan is drawn at 1/4" = 1'-0". A wall measures 3 1/2 inches on the print.
Real length = 3.5 in ÷ 0.25 in per foot = 14 feet.
A site plan at 1" = 20' shows a driveway 2.3 inches long: 2.3 × 20 = 46 feet.
Do not scale the drawing
Printing, copying and PDF zoom change the size. That is why many title blocks say "DO NOT SCALE DRAWING": use the written dimensions. Scaling is fine for a rough estimate; if a dimension is missing, ask for it rather than measuring the paper. A graphic (bar) scale printed on the sheet stays correct when the print is resized, so it is safer than the ratio for estimates.
The title block
Usually in the lower-right corner. Typical fields:
- Title (part or project name) and drawing number.
- Revision letter or number, plus a revision block listing what changed, when and who approved it.
- Scale, sheet (for example "2 of 5"), size (A, B, C, D or A4, A3...).
- Units (mm or inches) and general tolerances (lesson 3).
- Projection symbol (first- or third-angle).
- Material and finish for mechanical parts.
- Drawn by, checked by, approved by, with dates; on construction drawings, the engineer's or architect's seal where required.
Worked example: checking a print before you start
You are handed drawing 4410-202, revision C, sheet 1 of 2, scale 1:2, units mm, third-angle. Before you cut anything: confirm revision C is the current revision on the project's document list, find sheet 2, note that 1:2 means the part is twice the size you see, and note the general tolerance block.
Common mistakes
- Working from an old revision.
- Mixing up 1:2 and 2:1.
- Reading a 1/8" scale plan with the 1/4" side of the scale ruler.
- Missing a second sheet.
- Ignoring the units box (a 25.4 mm hole and a 1.000 in hole are the same; 25.4 inches is not).
Recap
- Line types tell you visible, hidden, centre, dimension, cutting plane and more; thick lines are outlines.
- Scales: ratios (1:2), architectural (1/4" = 1'-0"), engineering (1" = 20').
- Use written dimensions, not a ruler on the paper.
- The title block gives the number, revision, scale, units, projection and approvals.