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Engineering Drawing Line Types Reference (ASME Y14.2 Style)

Reading a drawing correctly starts with knowing what each line means. This quick reference lists the common line types, their weight and pattern, and how they are used. It is useful for new engineers, for supplier onboarding and for standardising drawing templates across a team.

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Line type	Weight	Pattern	Typical use
Visible (object) line	Thick	Continuous	Edges and contours of the part that can be seen in the view
Hidden line	Medium or thin	Short evenly spaced dashes	Edges and features behind a surface, only where they clarify the view
Center line	Thin	Long, short, long dash sequence	Axes of holes, cylinders and symmetrical features, and bolt circles
Dimension line	Thin	Continuous with arrowheads or ticks	Shows the extent of a dimension between extension lines
Extension line	Thin	Continuous, small gap at the object	Extends feature edges so a dimension can be placed outside the view
Leader line	Thin	Continuous with arrowhead	Points from a note, callout or symbol to the feature it describes
Cutting-plane line	Thick	Long dash with two short dashes, or evenly spaced dashes	Shows where a section view is taken, with arrows for viewing direction
Section line (cross-hatching)	Thin	Continuous parallel lines, usually at 45 degrees	Fills the cut surface in a section view, symbols can show material
Short break line	Thick	Freehand wavy line	Shows a short break in a long part to save drawing space
Long break line	Thin	Ruled line with freehand zigzags	Shows a long break on parts of constant cross section
Phantom line	Thin	Long dash with two short dashes, repeated	Alternate positions of moving parts, adjacent parts and repeated detail
Stitch line	Thin	Evenly spaced small dashes	Indicates a sewing or stitching path, and sometimes a hidden feature
Chain line	Thick	Long, short, long dash sequence	Indicates a surface or area that has a special requirement such as a treatment zone
Symmetry line	Thin	Center line with two short parallel lines at each end	Shows a view is shown as half of a symmetrical part

Line weights and patterns are summarised from common drafting practice such as ASME Y14.2. Absolute widths depend on your drawing standard and plotting setup. Check your company standard and the standard cited in the contract.

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How to use the table

Search for a line name such as phantom or cutting to jump to its row, or search for a use such as symmetry or section. Export the table to Excel or PDF and attach it to a drafting standard, a CAD training pack or a supplier drawing requirements package. The weight column helps you check that a template has distinct thick and thin line settings.

Use thick lines for visible edges, cutting planes and short breaks
Use thin lines for dimensions, extension lines, leaders and hatching
Add hidden lines only where they improve clarity

Where the conventions come from

Line conventions in US practice are set mainly by ASME Y14.2, with ISO 128 covering the same ground for metric drawings. Both define a small family of line types so that any trained reader interprets a drawing the same way. A common rule of thumb is that a thick line is about twice the width of a thin line. The aim is legibility after reduction, scanning and faxing or low resolution printing, so keep the weight contrast clear.

Common mistakes

Frequent problems include center lines drawn as plain dashes, hidden lines that clutter a view and make features ambiguous, cutting plane lines without direction arrows, and hatching that crosses a dimension without a break for the text. Another common issue is CAD layers where the line weight is set to the same value for everything, which makes the drawing look flat and hard to read after plotting. Review a sample print, not only the screen.

Frequently Asked Questions

What is the difference between a phantom line and a center line?

A center line marks an axis or symmetry and uses a long, short, long pattern. A phantom line uses a long dash followed by two short dashes and shows alternate positions, adjacent parts or repeated detail. They look similar, so check the pattern carefully.

How thick should thick and thin lines be?

Standards give minimum widths and a ratio, commonly thick being about twice thin. Typical values are around 0.6 mm for thick and 0.3 mm for thin, but your company standard or CAD template sets the exact values. Confirm with a test plot.

Should hidden lines always be shown?

No. Practice is to show them only where they make the drawing clearer. Too many hidden lines make a view hard to read, so a section view or an additional view is often a better choice.

Does ISO use the same line types?

ISO 128 defines a similar set with different naming and a defined set of line widths. Most basic types are equivalent, but codes, widths and terminology differ, so follow the standard your drawing cites.

If you manage drawing standards across several plants and want routine release checks tied to your PLM and ERP data, Netray can talk through options.

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