What is an OTF file?

OTF is a binary format, meaningful only to a program that knows it. It is used for typesetting and print.

The extension is .otf, and the full name is OpenType Font. Both matter less than what the file can hold, which is what the rest of this page is about.

Adobe published it in 1996. The specification is ISO/IEC 14496-22.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

OTF stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.

FontForge, Glyphs and FontLab read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Every current browser reads it.

That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.

OTF is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

OpenType is TrueType with a typographic layer on top, developed jointly by Adobe and Microsoft in the late 1990s to end a format war nobody was winning. The container is the same, which is why the two extensions install identically and work in the same places.

The addition is a set of tables describing what should happen to glyphs in combination: substitute this pair with a ligature, swap these figures for old-style ones, use the small-capital forms, apply this language’s rules for the letter i. A TrueType font maps characters to shapes. An OpenType font also knows how the shapes behave together, which is what makes real typesetting possible.

A well-made OTF carries several. Standard ligatures join fi and fl so the hook of the f does not collide with the dot. Old-style figures sit on the baseline with ascenders and descenders, and belong in running text where lining figures look like shouting. True small capitals are drawn at the right weight rather than being shrunk capitals. Tabular figures give every digit the same width so columns line up. Stylistic alternates offer a second version of a letter for a designer who wants one.

None of them appear by default in most software. Word exposes a small subset through the advanced font dialog and ignores the rest. Adobe InDesign and Illustrator expose nearly all of it. On the web, features are switched on in CSS. So a font that looks flat is usually a font whose features nobody has turned on — the letterforms are in the file either way.

An OTF usually holds cubic Bézier outlines inherited from Adobe’s Type 1 fonts, where a TTF holds quadratic ones. Cubic curves describe a given shape with fewer points, which is why type designers prefer drawing in them and why an OTF is sometimes slightly smaller than the same design as a TTF.

On screen the difference is nothing. Both are converted to pixels by the same rasteriser and neither is sharper. The one place it used to matter was printing to PostScript devices, where an OTF could pass its outlines through untouched. That is now a detail for people who run imagesetters.

The newest addition to OpenType is the variable font: one file containing a continuous range rather than a fixed weight. Instead of shipping Light, Regular, Medium, Bold and Black as five files, a variable font ships one and interpolates every weight between the extremes — and often other axes as well, such as width or optical size.

For the web the saving is substantial, because one file replaces five. Desktop support is patchier: most applications show a variable font as a handful of named instances rather than offering the slider, so the flexibility that justifies the format is often invisible outside a browser or a current Adobe application.

Because the programs are applying different feature sets. The same OTF in InDesign with ligatures and old-style figures on, and in a basic text editor with neither, produces visibly different text from identical characters. Nothing has been substituted and no fallback has occurred — one program is using the font’s tables and the other is not.

The same explains disappointment after a hand-off. A designer sets a specification using stylistic alternates and contextual forms; the client opens it in a program that ignores both, and concludes the font is wrong. It is worth knowing which features a layout depends on before that conversation.

Convert to WOFF2 rather than linking the OTF. It is the same font compressed for transport, typically thirty to fifty per cent smaller, and read by every current browser.

OpenType features survive the conversion — they are tables in the file, and compression does not remove them. What removes them is aggressive subsetting: a subsetter told to keep only the characters a page uses will happily discard the ligature and alternate tables too, and then the CSS switching them on does nothing. If a site loses its ligatures after optimisation, that is where to look.

When a foundry offers both, the practical advice is unexciting: take whichever the foundry recommends, and if there is no recommendation, take the OTF for print and design work and the TTF where an older application or a hinting-sensitive Windows workflow is involved.

What should not drive the decision is a belief that one extension is higher quality. The quality is in the drawing and in the feature tables, both of which the designer put there, and both of which either container can hold.