A path has no thickness

The line you drew does not exist

Draw a line in any vector program and give it a 2 mm stroke. You now believe there is a 2 mm-wide object on the page. There is not. There is a mathematical path, a curve of zero width, and an instruction attached to it that says when rendering, sweep a 2 mm-wide brush along me.

Nothing downstream is obliged to obey that instruction the way you expect, and the places where it goes wrong are the places this lesson exists for.

The parameters that actually change the shape

A stroke carries more than a width:

• Cap: what happens at an open end. butt stops exactly at the endpoint. round and square extend it by half the stroke width beyond. A 10 mm line with a 4 mm round cap measures 14 mm. This is why a dimension you carefully set comes out long.
• Join: what happens at a corner. miter extends both edges until they meet, round arcs across, bevel cuts the corner flat.
• Miter limit: the escape hatch on the above. At a sharp angle, a mitred join produces a spike that can run far beyond the corner; the limit is the ratio at which the renderer gives up and bevels instead. SVG and PDF default to 4, which cuts in at about 29°. A logo with a very sharp point can therefore render with a spike in one program and a flat corner in another, and both are following the specification.
• Dash array: a list of on/off lengths, in stroke units, plus an offset. Dashes are generated at render time; they are not objects.
• Alignment: centre, inside or outside. Illustrator, Affinity and CorelDRAW offer all three. SVG has only centre, so an inside-aligned stroke exported to SVG shifts by half its width. Inkscape approximates the other two with a Live Path Effect, which does not survive export to most other programs.

Hairlines, and the line that vanishes on press

CorelDRAW and several plotter drivers offer a width called hairline. It is not a measurement. It means the thinnest line this output device can produce.

• On a 600 dpi office laser, that is 1/600 inch = 0.042 mm. Visible, slightly weak, fine.
• On a 2400 dpi platesetter, that is 1/2400 inch = 0.011 mm. That is thinner than a press can hold ink. The line breaks into dots, or disappears.

So a hairline keyline proofs perfectly on the printer beside your desk and is missing from five thousand cartons. It is one of the classic print failures, and the mechanism is entirely in the definition of the word.

Use real numbers instead. Practical minimums, worth asking your supplier to confirm:

• Offset litho, positive line on white: 0.1 mm (about 0.25 pt)
• Offset litho, reversed out of a solid: 0.15 mm, because ink spreads into the gap
• Digital toner and inkjet: around 0.08 mm
• Screen print: 0.3 mm, coarser on textile
• Vinyl cutting: 1 to 2 mm, because a human has to weed it
• Embroidery: 1.2 mm minimum satin width

Scale strokes, or do not

Every program has a preference deciding whether a stroke scales with the object. Both behaviours are useful and the wrong one is expensive.

• On is what you want for a logo. Shrink the mark to 10 mm and the outline shrinks with it.
• Off is what you want for a technical drawing or a die-line, where 0.25 mm means 0.25 mm at any zoom.

Illustrator: Preferences ▸ General ▸ Scale Strokes & Effects. Inkscape: the four toggle buttons at the right end of the selection tool’s control bar: the first one governs stroke width. CorelDRAW: Scale outline with object in Object Properties.

The failure is always the same direction. Someone designs a mark at 200 mm with a 2 mm outline, scales it to a 15 mm badge with scaling off, and the outline is now proportionally thirteen times too heavy. It looks like a blob and nobody can say why.

When to make the stroke real

Stroke to path converts the instruction into geometry: a closed outline you can boolean, cut, engrave or stitch. It is destructive: the centreline is gone, and so is the ability to change the width by typing a number.

Do it when the file is leaving for a machine, and keep the editable original. Do not do it early, because every later size change then means redrawing.