Comment 23 for bug 448286

Since SVG doesn't actually have hairline support, the best way I can think of to implement it in Inkscape is to set width=1px, vector-effect="non-scaling-stroke", and a new inkscape:vector-effect="hairline" property.

On screen, width=1px and vector-effect="non-scaling-stroke" imitates hairlines.

It doesn't necessarily work that way in PDFs or on printers. In PDF and Postscript, a hairline should have a width of 0. In the Windows print API, they should have a width of 1 and be drawn with the PS_COSMETIC pen. In these cases (printing or saving as PDF), drawing inkscape:vector-effect="hairline" lines with a width of 0 is reasonable. Cairo might require some small patches to treat these reasonably, but there seems to be some desire over on the Cairo team to handle hairlines consistently.