The TTF file (.ttf)

A TTF file holds one font: the outline of every letter, digit and symbol it covers, how wide each one is, how far apart lines sit, which character each shape belongs to, and the font's name and licence. Operating systems, word processors, design apps and browsers use those outlines to draw text at any size, on screen or on paper.

Apple developed TrueType in the late 1980s as an alternative to Adobe's PostScript Type 1 fonts, and released it in 1991. Microsoft licensed it for Windows 3.1, and it became the standard font format on both systems. Most fonts that come with Windows and macOS are TrueType or its successor, OpenType.

Today a .ttf file is, technically, an OpenType font with TrueType outlines. The OpenType specification, which Microsoft and Adobe built on top of TrueType from 1996 onwards, describes both .ttf and .otf files. The extension tells you which kind of outlines to expect, not which standard applies.

A TTF file is a small database. The header lists tables, each identified by a four-letter tag, and each table does one job. A font needs these to work:

Outlines. TrueType glyphs are drawn with straight lines and quadratic Bézier curves: each curve segment has one control point, against two in the cubic curves of PostScript and CFF fonts. A point is either on the outline or an off-curve control point, and two control points in a row imply an on-curve point halfway between them. Quadratic curves need more points for the same shape but are fast to draw.

Hinting. At small sizes on a screen, a letter is only a few pixels tall, and an outline scaled down blindly turns into a blur. TrueType fonts can carry instructions, small programs in a bytecode language, that nudge points onto the pixel grid so stems stay sharp and even. Good hinting is skilled, slow work; many modern fonts are hinted automatically or not at all, because high-resolution screens and modern rendering need it less. Our samples are unhinted.

For installing on a computer, TTF and OTF are equally good: both support the same layout features, scripts and variable axes. Pick whichever the font maker supplies. For a website, convert to WOFF2 rather than serving the TTF: it's the same font at a fraction of the size. Two older formats you may still meet: .ttc, a TrueType collection holding several fonts that share tables, and .dfont, a TrueType font in the old Mac data-fork format.

Since OpenType 1.8 in 2016, a single TTF can hold a continuous range of styles. A variable font stores one set of outlines plus tables describing how each point moves along design axes such as weight (wght), width (wdth) or slant (slnt). The main extra tables are fvar (the axes and named styles), gvar (how the outlines change) and STAT (how the styles are named).

Our sample-variable.ttf has one axis, weight, from 300 to 700, with five named styles: Light, Regular, Medium, SemiBold and Bold. Apps with variable-font support show a weight slider or the named styles; on the web, font-weight: 300 700 in @font-face unlocks every weight in between.

• Windows: double-click the file to preview it in Font Viewer, then click Install. Right-click offers Install for all users. Fonts live in C:\Windows\Fonts, or in your user folder if installed only for you.
• macOS: double-click to open it in Font Book and click Install. Font Book also checks fonts for problems (File, Validate File).
• Linux: copy it to ~/.local/share/fonts and refresh the font cache, as below.
• On a website: convert it to WOFF2 and load it with @font-face. Browsers accept TTF directly too, but it's bigger.
• Convert: fontTools (pyftsubset, ttx, or a few lines of Python) and FontForge convert between TTF, OTF, WOFF and WOFF2; Google's woff2_compress turns a TTF straight into WOFF2.
• Phones: iOS and Android don't install loose font files by double-tapping. On iOS, fonts come through apps or configuration profiles; on Android, through the manufacturer's theme app where available, or within individual apps.

# Linux: install a font for your user, then refresh the font cache
mkdir -p ~/.local/share/fonts
cp sample-regular.ttf ~/.local/share/fonts/
fc-cache -f
fc-list | grep "99helpers"

• It's installed but not in the font menu. Most apps read the font list when they start: close and reopen the app. On Linux, run fc-cache -f.
• "Not a valid font". The file is damaged, incomplete (an interrupted download), or not really a TTF: a renamed WOFF or WOFF2 file, or an HTML error page saved as .ttf. Check the first four bytes; they should be 00 01 00 00.
• Boxes instead of letters. The font doesn't contain those characters. Boxes, often called tofu, are the missing-glyph shape (glyph 0, .notdef). Our sample-digits-subset.ttf shows this for every letter.
• Fuzzy at small sizes on Windows. An unhinted or lightly hinted font can look soft at body-text sizes on lower-resolution screens. Try a hinted version of the font, or a size where it renders well.
• Two versions conflict. If an old and a new version of a font have the same name, apps may pick the wrong one. Remove the old version and restart the app.
• Bold or italic looks wrong. If the family has no real bold file installed, apps fake it by thickening the letters. Install the bold font too.

A font file is software, and comes with a licence. Fonts bundled with an operating system are licensed for use on that system, not for redistribution or for putting on a website. Open-source fonts, usually under the SIL Open Font License, can be used, embedded and shared freely. Commercial fonts often need separate licences for desktop, web, app and e-book use.

The font itself records some of this. The name table carries copyright and licence text, and the OS/2 table's fsType field states the embedding permission: installable, editable, preview and print, or restricted, plus flags forbidding subsetting or allowing bitmap-only embedding. Word processors and PDF tools read fsType when deciding whether to embed a font in a document.