TTF, OTF, WOFF, WOFF2

File extensions are convention, not specification. .ttf and .otf can be the same container holding different outlines; .woff and .woff2 are not new formats at all but compressed wrappers for the web.

Read the header, not the extension#

The first four bytes (sfntVersion) say what the file really is:

So neither "starts with OTTO means it must be .otf" nor ".otf can never hold TrueType outlines" is true — the convention says so, the spec does not.

TrueType and CFF: two outline flavours#

The real difference is how outlines are stored:

• TrueType: quadratic Béziers, in glyf plus loca;
• CFF / CFF2: cubic Béziers, in CFF (note the trailing space) or CFF2.

Both live inside the same table framework, so name, OS/2 and cmap look identical. Which is why the same design exported in both formats keeps almost everything except the outlines.

Font collections#

.ttc / .otc pack several faces of one family into a single file, sharing parts of cmap and name to save space. Common among system fonts (many macOS system fonts, for instance).

To software this is not "one font" but a container: pick a face by index. When this site meets a collection, it prefers the face matching the requested family.

WOFF and WOFF2#

Both are containers: the original font, compressed, plus metadata and private-data slots. Unwrapped, what remains is an ordinary TrueType/OpenType font.

• WOFF: zlib compression;
• WOFF2: Brotli, plus a restructured table layout — substantially smaller.

Browsers support WOFF2, with older ones falling back to WOFF. Shipping WOFF2 first is not laziness; it is genuinely smaller.

Variable fonts are extra tables, not a format#

A variable font has the same extension as a static one; what differs is the presence of fvar (axes), gvar or CFF2 (per-axis deltas) and STAT (style attributes). So:

• a .ttf may contain endless weights (fvar + gvar);
• or just one (static).

The extension cannot tell you; the table directory can.

This site's position#

This site does not host or offer downloads of font files (see docs/adr/0001): the server keeps fingerprints and metadata. What you see here is "this font exists, this is what it looks like, these are its metrics" — not "click to download".

Loading a library font in the inspector fetches its bytes from the upstream CDN into browser memory for temporary registration; nothing lands in this site's storage.

Further reading#

• OpenType spec: the sfnt container, glyf and CFF
• W3C: the WOFF and WOFF2 specifications
• OpenType spec: fvar, gvar, STAT