Lines, sections and details: line types, section views and detail views
Learn the ISO 128 line types and widths, how to read and draw a section view, which parts are never hatched, and how to label a detail view with its scale.
After this lesson you can:
- Identify each ISO 128 line type on a drawing and state what it represents
- Apply the line width series and the 2:1 wide to narrow ratio
- Read a cutting plane and its labels and draw the matching section view
- Choose between full, half, offset, revolved, removed and local sections
- Decide which features are not hatched when cut lengthwise
- Call up a detail view with a letter, a boundary circle and a scale
- Estimated time
- 15 minutes
- Before you start
- Lesson 2: First angle vs third angle projection
A drawing is a code written in lines. Before you can read a single dimension, you have to know which lines are edges, which are hidden and which are axes. Some lines are only there to carry a number. The same code tells the machinist where you cut the part open to show the inside. It also shows which small corner you blew up at 5:1 because the groove matters.
Graduates often let the CAD system make every choice. That works until a checker returns the drawing for a hatched bolt, a hidden line through a dimension or a detail view with no scale. ISO 128-2:2020 governs lines, and ISO 128-3:2022 governs views and sections. Where US practice uses a different name, the ASME Y14.3-2012 (R2024) term is given in brackets.
Types of lines in engineering drawing
ISO 128-2:2020 replaced the older ISO 128-20 to 128-25 parts and now holds the line rules in one document. It defines two things you use daily: the width series and the line types.
The width series is 0.13, 0.18, 0.25, 0.35, 0.5, 0.7, 1, 1.4 and 2 mm. Each step is about the square root of 2 larger than the last. A drawing enlarged or reduced from A3 to A2 or A4 therefore keeps a legible set of widths. A drawing uses one pair, with wide lines twice the width of narrow lines (2:1). Typical pairs are 0.5 mm and 0.25 mm, or 0.7 mm and 0.35 mm. An extra wide line (4:2:1) exists but is rare in mechanical work. Parallel lines should never be closer than 0.7 mm, or they merge in print.
| ISO 128-2 type | Line | Width | Mechanical use (ISO 128 convention) | US name |
|---|---|---|---|---|
| 01 | Continuous | Wide | Visible outlines and edges | Visible line |
| 01 | Continuous | Narrow | Dimension, extension (projection), leader lines, hatching, short centre lines | Dimension, extension, leader line, section lining |
| 01 | Continuous, freehand or zigzag | Narrow | Limits of partial views, local sections and breaks | Break line |
| 02 | Dashed | Narrow | Hidden outlines and edges | Hidden line |
| 04 | Long-dashed dotted | Narrow | Centre lines, lines of symmetry, pitch circles | Center line |
| 04 | Long-dashed dotted, wide at ends and changes of direction | Narrow | Cutting planes | Cutting-plane line, viewing-plane line |
| 05 | Long-dashed double-dotted | Narrow | Adjacent parts, extreme positions of moving parts, initial outlines before forming | Phantom line |
US drawings use the same set, calling wide and narrow “thick” and “thin”.
Which line wins when lines coincide
Two lines often fall in the same place: a hidden bore edge behind a visible face, or a centre line along a hidden hole. You draw only one, in this order of priority:
| Priority | Line kept |
|---|---|
| 1 | Visible outline (continuous wide) |
| 2 | Hidden outline (dashed narrow) |
| 3 | Centre line (long-dashed dotted narrow) |
| 4 | Dimension, extension and hatching lines give way to all of the above |
Centre lines should extend a short distance beyond the feature and should cross at long dashes, not at gaps or dots. Hidden lines are a tool, not a duty: show them only where they remove doubt. If a section would show the feature more clearly, use a section.
On the job
A common first-drawing comment is “too many hidden lines”. CAD makes it easy to switch hidden lines on for every view, and the result is a web of dashes that nobody can dimension from. A checker expects hidden lines only where a feature cannot be seen in any other view, and never dimensioned to directly. If you find yourself adding a third hidden line to explain a bore, delete them and add a section. The machinist will thank you: dimensions to a sectioned edge are unambiguous.
How to read a section view
A section view shows the part as if it had been cut open along a plane and the near piece removed. ISO 128-3:2022 covers views, sections and cuts. It is neutral between first and third angle, so the same section rules apply on both sides of the Atlantic.
The cutting plane is drawn on the parent view as a long-dashed dotted narrow line, thickened at the ends and wherever it changes direction. Arrows at the thick ends point in the direction of viewing, and capital letters beside the arrows name the section. The section itself is labelled with the same letters, for example A-A (US: SECTION A-A). Label every section; most checkers insist.
Surfaces the plane cuts through are hatched with continuous narrow lines. Everything visible behind the plane is drawn as normal visible outline. Hidden lines behind the plane are left out unless they are essential.
Types of section view
| Section | US name (ASME Y14.3) | What it is | Use it for |
|---|---|---|---|
| Full section | Full section | One straight plane through the whole part | Turned parts, housings, anything whose inside is the point |
| Half section | Half section | One half sectioned, one half external, divided by the centre line | Symmetrical parts where you need inside and outside in one view |
| Section in two or more planes | Offset section | Stepped cutting plane through features not on one line; steps are not shown in the section | Holes and bosses scattered across a plate |
| Revolved section | Revolved section | Cross-section rotated 90 degrees and drawn on the view itself in narrow outline | Arms, spokes, ribs, rolled sections |
| Removed section | Removed section | Cross-section drawn clear of the view, labelled, often at larger scale | Shafts with keyways and flats, where the view is crowded |
| Local section | Broken-out section | A small area cut away, bounded by a freehand or zigzag break line | One tapped hole or groove in an otherwise solid view |
In a half section, the external half carries no hidden lines: the sectioned half already shows the inside. The dividing line is the centre line, not a visible outline, because the part is not actually split there.
What is not hatched in a section
Some features are left unhatched when the cutting plane runs along their length, even though the plane passes through them. The rule is conventional: hatching them adds no information and makes the drawing harder to read.
| Feature cut lengthwise | Drawn as | Reason |
|---|---|---|
| Shafts, spindles, axles | Unhatched, external outline | Solid; the section would show nothing new |
| Bolts, screws, studs, nuts, washers | Unhatched | Standard parts, defined by their callout |
| Pins, rivets, keys, cotters | Unhatched | Solid, and small hatched areas turn black in print |
| Ribs and webs | Unhatched outline | Hatching would make a thin rib look like a solid block |
| Spokes of wheels and pulleys | Unhatched | Would read as a solid disc |
| Balls and rollers in bearings | Unhatched | Solid rolling elements |
If the same parts are cut across (perpendicular to their axis), they are hatched normally. A shaft in a removed section is hatched; a shaft in a longitudinal assembly section is not.
Hatching rules
Hatching is a continuous narrow line, normally at 45 degrees to the main outline or axis of the section. One part keeps the same direction and spacing in every section on the sheet. Adjacent parts are hatched in the opposite direction, or at a different spacing where three or more parts meet. Hatching stops for dimension text rather than running through it, and large areas may be hatched only along the boundary. If the outline itself runs at 45 degrees, pick another angle.
Worked example: which parts are hatched?
A longitudinal section runs through the axis of a small drive. It shows a steel shaft, the key, and a pulley with a solid central disc keyed to the shaft. A hex nut and washer sit on the shaft end. The section also cuts a deep-groove ball bearing and the cast housing, including a stiffening rib cut along its length.
| Component | Hatched? | Direction and reasoning |
|---|---|---|
| Shaft | No | Cut lengthwise, solid |
| Key | No | Cut lengthwise, solid |
| Nut and washer | No | Standard fasteners |
| Pulley hub and solid disc | Yes | 45 degrees one way; a continuous disc is not a spoke |
| Bearing inner and outer rings | Yes | Opposite directions to each other and to their neighbours |
| Bearing balls | No | Rolling elements |
| Housing wall | Yes | Opposite direction to the outer ring it touches |
| Housing rib | No | Drawn in outline so it does not look like solid wall |
Check the boundaries: every pair of hatched parts in contact must differ in direction or spacing. The outer ring and housing are in contact; the pulley and housing are not. If the housing and pulley both run at 45 degrees one way, that is acceptable because they do not touch.
On the job
CAD assembly sections hatch everything the plane cuts unless you tell them not to. The result: bolts, dowels and shafts filled with hatching, and a checker’s red pen. Most systems let you mark components as “do not section” once in the assembly. Many companies set fasteners and shafts to that by default in the library. Check the setting before you issue, and check that two touching parts have not ended up with identical hatching after a rebuild.
Detail views and enlargement scale
A detail view enlarges a region too small to dimension at the main scale, such as a groove, an undercut or a thread runout. The parent view marks the region with a continuous narrow circle (or a rounded outline) and a capital letter. The enlarged view carries the same letter and its scale, for example B (5:1), or in US practice DETAIL B SCALE 5:1.
Use a preferred scale from ISO 5455:1979: enlargements of 2:1, 5:1, 10:1, 20:1 and 50:1. The main scale goes in the title block; every view that differs from it must state its own. BS 8888:2025, the UK framework standard for technical product documentation, calls up these ISO conventions, so a UK drawing follows the same rules.
On the job
The classic detail view fault is a missing or wrong scale. The CAD label often defaults to the sheet scale, or the view was rescaled late and the label never updated. A supplier who measures off a print (they should not, but some do) will then make the groove five times too small. Before issue, check each detail and removed section. The letter must match the circle, and the scale must be a preferred ISO 5455 value. The view must actually be drawn at that scale.
Where this leads next
Next, Lesson 4 puts numbers on those views and explains what an untoleranced dimension actually means. For a tapped hole shown in a local section, the thread engagement length calculator checks that the drawn depth carries the load. For the projection rules that decide where each section is placed, go back to Lesson 2.
Check your understanding
5 questions. Choose an answer, then check it. Nothing is saved.
Key terms
- Continuous wide line
- The heaviest line in normal use, reserved for visible outlines and edges of the part.
- Hidden line
- A dashed narrow line showing an edge or outline that is behind material in the chosen view.
- Cutting plane
- A long-dashed dotted narrow line, thickened at the ends and at changes of direction, with arrows and letters showing where a section is taken and which way it is viewed.
- Hatching (US: section lining)
- Continuous narrow lines, normally at 45 degrees, filling the surfaces that the cutting plane passes through.
- Half section
- A view of a symmetrical part drawn half in section and half as an external view, split on the centre line.
- Local section (US: broken-out section)
- A small section bounded by a freehand or zigzag break line, used to show one internal feature without sectioning the whole view.
- Detail view
- An enlarged view of a small area, identified by a circle and a capital letter on the parent view and labelled with the same letter and its scale.
Standards referenced
| Standard | Edition | Title | Used here for |
|---|---|---|---|
| ISO 128-2 | 2020 | Technical product documentation, General principles of representation, Part 2: Basic conventions for lines | Line types, width series and the wide to narrow ratio |
| ISO 128-3 | 2022 | Technical product documentation, General principles of representation, Part 3: Views, sections and cuts | Cutting planes, section types, hatching and detail views |
| ASME Y14.3 | 2012 (R2024) | Orthographic and Pictorial Views | US terms for section views, section lining and viewing planes |
| ISO 5455 | 1979 | Technical drawings, Scales | Preferred enlargement scales for detail views |
| BS 8888 | 2025 | Technical product specification and documentation | UK framework that calls up the ISO drawing conventions |
Editions checked on 29 Sep 2026. Standards are copyright documents; this lesson explains and paraphrases them. Buy or borrow the current edition before relying on a value.