Types of Lines in Technical Drawing (ISO 128) and Line Widths

In a technical drawing every line carries meaning. A thick line and a dashed line drawn in the same place describe two completely different things — one an edge you can see, the other an edge hidden inside the part. Before you can read a drawing you have to be able to read its lines.

Line types are standardized in ISO 128 (TS 88). There are eight in everyday use. Learn them once and they never change, on any drawing, anywhere.

The line types

Figure 2.1 — Line types used in technical drawing, where each is used and how wide it is

Line widths

Only two widths are used on a drawing: a thick one and a thin one. The thin line is always half the thick line. If you choose 0,7 mm for thick, thin is 0,35 mm; if thick is 0,5 mm, thin is 0,25 mm.

Choose the width pair according to the sheet size — heavier on A2 and larger, lighter on A4.
Once chosen, keep it constant. The same line type must look the same everywhere on the sheet.
Dash and gap lengths must also be uniform: dashes about 3 mm, gaps about 1 mm on A4 work.

The three lines you will use most

Ninety per cent of a normal drawing is made of just three line types: continuous thick for visible edges, dashed for hidden edges, and chain for center lines. The figure below shows all three on a single part.

Figure 2.2 — Continuous thick, dashed and chain lines used together on the same part

The same feature, two different lines. A hole seen from above is a continuous thick circle — you are looking straight into it. The same hole seen from the front is two dashed lines, because its walls are inside the material. Nothing about the hole changed; only your viewpoint did.

Rules for dashed lines

A dashed line starts and ends with a dash, never with a gap.
Where a dashed line meets a visible line, they touch — no gap is left.
Where two dashed lines meet at a corner, the dashes meet at the corner.
Dashes are all the same length; do not let them drift longer as you draw.

Rules for center lines

A center line starts and ends with a long dash, and extends 2–3 mm past the outline.
Two center lines cross at their long dashes, not at the dots — the crossing marks the center.
Every circle and every cylinder gets a center line. A circle without one is an incomplete drawing.
Symmetrical parts get a center line on the axis of symmetry.

What a dashed line is telling you

Something is inside. A hole, pocket, slot or step exists but cannot be seen from this direction.
Look at another view. The true shape of that feature is drawn with a continuous line in another view or in a section.
Expect a section. If a view is full of dashed lines, the draughtsman has almost certainly added a section view — find it.
Dimensions are not placed on dashed lines. The true size of a hidden edge is given where that edge is visible, or in the section.

When lines coincide: the priority order

Sometimes two different lines fall on exactly the same place on the paper. Only one can be drawn, and the choice is not free — there is a fixed priority:

Continuous thick — visible edge
Dashed — hidden edge
Chain — center line / cutting plane

Figure 2.3 — The priority order applied when lines coincide

A visible edge always wins. If you can see it, you draw it thick — whatever else happens to be underneath.

Lettering

Writing on a drawing is also standardized (ISO 3098 / TS 88-20). The aim is that anyone can read the figures without hesitation — a 3 that can be mistaken for an 8 is a scrap part waiting to happen.

Two styles are defined: type A (narrow) and type B (wider, the usual choice).
Letters may be upright or sloped 75° to the horizontal — but not mixed on one sheet.
Standard character heights: 2,5 – 3,5 – 5 – 7 – 10 – 14 – 20 mm. Each step is √2 times the one before.
Dimension figures are usually 3,5 mm; titles and view labels are larger.
The line width of the lettering is h/10 for type B (h = character height).
Never write dimension figures on top of a line, and never let a line cross a figure.

Upright (90 degree) and sloped (75 degree) standard lettering according to TS EN ISO 3098-1 — Upright lettering is written at 90°, sloped lettering at 75° (ISO 3098-1). Turkish text: dik yazı = upright, eğik yazı = sloped

Lettering proportions

Every lettering dimension is derived from the capital height h. Nothing smaller than 2,5 mm is used, because it becomes unreadable after photocopying or scanning.

Term	Symbol	Formula	Example: h = 5 mm
Capital height	h	chosen value	5 mm
Lower-case height	c	7/10 × h	3,5 mm
Spacing between letters	a	2/10 × h	1 mm
Line spacing	b	14/10 or 16/10 × h	7 – 8 mm
Stroke width, type A	d	1/14 × h	0,36 mm
Stroke width, type B	d	1/10 × h	0,5 mm

Type B sloped lettering is the usual choice in mechanical drawings: it is easier to read and stays tidier when written by hand.

Test yourself

Which line shows a hidden edge?
Dashed thin.
The thick line on a drawing is 0,7 mm. How wide is the thin line?
0,35 mm — always half.
A hidden edge and a center line fall on the same place. Which is drawn?
The hidden edge — dashed beats chain.
Why does a chain line extend past the outline of a circle?
So that the center is clearly marked and can be used as a reference for dimensioning.
What does a chain line with thick ends mean?
It is the trace of a cutting plane — the drawing contains a section view.
What is the standard height of a dimension figure?
Usually 3,5 mm, chosen from the standard series 2,5 / 3,5 / 5 / 7 / 10 mm.
An edge on a drawing is both thick and dashed. What does it mean?
Nothing — it is a drawing error. An edge is either visible or hidden, never both.

Next lesson. With the sheet set up and the lines understood, Lesson 3 covers the geometric constructions every draughtsman needs: bisecting angles, dropping perpendiculars, tangents, regular polygons and the ellipse — all with compass and straightedge only.