SLF Font File Formats Explained: TTF, OTF, OPF, HLF, and SVG
If you have bought a single line font on Etsy or another marketplace, you have probably seen a bundle containing files labelled SLF TTF, OPF OTF, HLF TTF, and SVG — sometimes all four. They all produce single strokes on your machine, but they are not the same thing. This guide explains what each format actually is, how the "single line trick" works inside a standard font file, and which format you should reach for depending on your workflow.
Why there are so many formats
The short answer is that no single open standard for single line fonts exists. TTF and OTF were designed for screen and print — filled, closed outlines. Laser engravers and pen plotters need open paths, not fills. Font tool developers and Etsy sellers have each worked around this limitation in slightly different ways, and the names they chose — SLF, OPF, HLF — are marketing labels, not technical standards. Under the hood, they all use the same encoding trick, just with different curve types and varying levels of craftsmanship.
TTF and OTF: what is actually different
TTF (TrueType Font) and OTF (OpenType Font) are both installable font formats that work across Windows, macOS, Linux, and most design software. The key technical difference is the curve type. TTF uses quadratic Bézier curves, which require more points to represent a smooth arc. OTF uses cubic Bézier curves, which can represent the same arc with fewer points and higher precision. In practical terms, OTF produces cleaner curves with lower node counts — which is exactly what you want for laser and plotter work, where every extra node means an unnecessary direction change. OTF also supports the full OpenType feature set: pair kerning, ligatures, alternate glyphs, contextual substitution. TTF supports these too in modern implementations, but OTF is the better container for fonts that take full advantage of typography features.
The zero-width contour trick: how SLF fonts work inside TTF and OTF
TTF and OTF technically require every glyph to be a closed outline — a filled shape, like the "A" you would see in a printed book. A single line has no fill and no closed outline. The solution used by every SLF, OPF, and HLF font on the market is to encode the open stroke as a closed outline where the two sides sit exactly on top of each other. Imagine folding a piece of string in half so both ends meet: you have a path that travels out along the stroke and immediately doubles back along the same route. The result is a zero-width closed contour that looks and behaves like a single open path to any machine that reads the path coordinates. LightBurn, Inkscape, AxiDraw, and EZCad all interpret this correctly as a single stroke with no double-cut.
SLF TTF and OPF OTF: different names, same concept
SLF TTF (Single Line Font, TrueType container) and OPF OTF (Open Path Font, OpenType container) describe the same encoding approach — the zero-width contour trick described above — in different font containers. Sellers who package their fonts as SLF TTF have chosen the TTF container, typically for maximum compatibility with older software. Sellers who use OPF OTF have chosen the OTF container for better curve precision and feature support. When you see both in a bundle, they contain the same letterforms — the OTF version will generally have smoother curves and support richer typography features if the designer took advantage of the format.
HLF TTF: the Hershey lineage
HLF stands for Hershey Line Font. The Hershey fonts were created in 1967 by Dr. A.V. Hershey at the US Naval Weapons Laboratory, originally as stroke data for pen plotters attached to early computers. They were the first systematically designed single-stroke typefaces, and their data has been in the public domain for decades. When sellers offer HLF TTF files, they have taken the original Hershey stroke data and packaged it in a TTF container using the same zero-width contour trick. The aesthetic is distinctive: geometric, engineering-drawing precision, reminiscent of CAD annotations or early computer graphics. If you want a font that looks handcrafted or calligraphic, HLF is not it. If you want a clean, neutral technical font with proven plotter geometry, HLF is a solid choice.
SVG: the odd one out
SVG files in a font bundle are not an installable font. They are a static artwork file — typically a "character sheet" with every glyph laid out as individual path elements in a grid. You open the SVG in Inkscape, Cricut Design Space, or Affinity Designer, select the letter you need, copy its path, and place it manually in your design. There is no typing, no automatic spacing, and no kerning — you are assembling letters one by one. This is genuinely useful for Cricut users who want to cut a single word and do not need the overhead of installing a font. It is also a useful reference for anyone who wants to inspect or modify individual glyph paths. But if your workflow involves composing multi-line text or you use the same font repeatedly, installing the OTF or TTF and using the software text tool will save you significant time.
Typography features: what each format actually supports
Three features separate a professionally made SLF font from a basic one: pair kerning, ligatures, and glyph variations. Pair kerning adjusts the spacing between specific letter combinations — the classic example is "AV", where the diagonal strokes would leave a visible gap without a negative kern. Ligatures replace specific character sequences with a single combined glyph — for a single line font, "fi" as a ligature means one continuous stroke rather than two strokes that nearly collide. Glyph variations provide alternate letterforms for the same character — a more casual "a" alongside the standard form, for example. OTF supports all three through its OpenType layout tables (GPOS for kerning, GSUB for ligatures and alternates), with class-based kerning that can handle hundreds of pairs compactly. TTF supports them too in modern implementations. SVG files as described above support none of these features. The HLF fonts, being converted from 1967 stroke data, typically lack kerning and ligatures entirely.
Which format to use for your workflow
If your workflow is: type text in LightBurn, Inkscape, Affinity Designer, or CorelDRAW and send to your machine — install the OTF. It gives you the best curve quality, full kerning, ligatures, and alternate glyphs. If your software only accepts TTF, use the TTF. If you are a Cricut user composing a single short word in Design Space — use the SVG character sheet. If you need a neutral technical font for engineering labels, badges, or serial numbers and want proven plotter geometry — HLF TTF is a reliable choice. If you compose your text in a dedicated tool like Pathhaus Studio and export the result as an optimised SVG, DXF, HPGL, or G-Code file — the font format question becomes irrelevant: the output file already contains the paths your machine needs, with travel-optimised ordering and calibrated node counts.
What Pathhaus fonts include
Every font in the Pathhaus library is built as a true single-stroke open-path font designed from the outset for machine use — not a screen font that has been converted. The library fonts are stored in a format that carries the full glyph set, pair kerning data, ligatures, and glyph variations together in one file. When you compose text in Pathhaus Studio and export, the output SVG, DXF, HPGL, or G-Code file contains only the exact paths your machine needs: no double strokes, no closed outlines, travel-optimised path order. Downloadable OTF files let you install Pathhaus fonts as system fonts and use them directly in LightBurn, Inkscape, or any other application — with all kerning, ligatures, and alternate glyphs intact.
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