What is an OTF file?
OpenType Font (OTF) is a scalable font format jointly developed by Microsoft and Adobe, building on Microsoft’s TrueType format and Adobe’s Type 1 PostScript technology. Released in 1996 and registered as an ISO standard (ISO/IEC 14496-22), OpenType unified the previously fragmented world of font formats. It supports both PostScript cubic Bézier outlines (CFF — the OTF variant) and TrueType quadratic spline outlines (the TTF variant), making it universally cross-platform.
OTF’s defining advantage over TTF is its advanced typographic layout features — ligatures, alternates, small caps, ordinals, fractions, contextual substitutions, and more — enabling sophisticated, professional typography that adapts to language and context.
How to open OTF files
- Font Viewer (Windows) — Double-click to preview the font and install
- Font Book (macOS) — Built-in font manager and installer
- FontForge (Windows, macOS, Linux) — Free, professional font editor
- Adobe InDesign / Illustrator (Windows, macOS) — Full OpenType feature access for print layout
- Glyphs (macOS) — Professional font design application
Technical specifications
| Property | Value |
|---|---|
| Outline format | CFF (cubic Bézier, PostScript) — the “true” OTF; or TrueType (quadratic splines) |
| Layout features | OpenType Layout — GSUB (glyph substitution) + GPOS (glyph positioning) |
| Script support | Multi-script, multi-language (Latin, Arabic, Devanagari, CJK, etc.) |
| Variable fonts | OpenType 1.8+ variable font support (2016) |
| Maximum glyphs | 65,535 |
| Magic bytes | 4F 54 54 4F (OTTO for CFF-based OTF) |
| MIME type | font/otf |
OpenType features
OpenType’s layout engine enables typographic refinements that automatic systems apply based on language and context:
| Feature code | Name | Example |
|---|---|---|
liga | Standard ligatures | fi, fl, ffi → combined glyphs |
dlig | Discretionary ligatures | ct, st → decorative alternatives |
smcp | Small capitals | lowercase → SMALL CAPS |
onum | Oldstyle numerals | 1234 → proportional oldstyle figures |
frac | Fractions | 1/2 → ½ |
sups | Superscripts | 2nd → 2ⁿᵈ |
calt | Contextual alternates | Adaptive glyph selection by context |
kern | Kerning | Precise letter-spacing adjustments |
In CSS, OpenType features are enabled via font-feature-settings:
h1 {
font-feature-settings: "liga" 1, "kern" 1, "smcp" 1;
}Or the higher-level shorthand:
body {
font-variant-ligatures: common-ligatures;
font-variant-numeric: oldstyle-nums;
}Common use cases
- Professional typography: Print and digital publications where advanced features like ligatures, small caps, and oldstyle numerals are required
- Multi-language publishing: Single OTF files supporting Arabic, Latin, Greek, and Cyrillic scripts simultaneously
- Variable fonts: OpenType 1.8 variable fonts embed multiple weights, widths, and optical sizes in a single file
- Web and print: OTF works in browsers (via WOFF/WOFF2 conversion), desktop publishing apps, and word processors
- Icon fonts: Many icon libraries distribute as OTF for desktop use
OTF vs TTF
| Feature | OTF (CFF) | TTF |
|---|---|---|
| Outline format | PostScript cubic Bézier | Quadratic splines |
| OpenType features | ✅ | ✅ (both use same OT tables) |
| File size | Often slightly smaller | Slightly larger |
| Rendering (old) | Better on macOS/Linux | Better on Windows (ClearType) |
| Rendering (modern) | Equivalent | Equivalent |
| Variable fonts | ✅ | ✅ |
Both OTF and TTF support the full OpenType feature set — the difference is only the outline format. Modern rendering engines handle both equally well. For web use, both are converted to WOFF2 anyway, making the distinction less relevant.
Installing OTF fonts
- Windows: Double-click the
.otffile → “Install” button; or right-click → “Install for all users” - macOS: Double-click → “Install Font” in Font Book
- Linux: Copy to
~/.fonts/or/usr/share/fonts/and runfc-cache -f
After installation, the font appears in all applications that use the system font list.