OpenType file signature
Magic number at the start of the file · OTTO
An OpenType font with PostScript (CFF) outlines starts with the tag OTTO. OpenType fonts that use TrueType outlines instead start with 00 01 00 00, like a plain TrueType file.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 4F 54 54 4F (the text OTTO), it is a OpenType file.
Formats that use the 4F 54 54 4F signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.otf
- hexdump -C -n 16 example.otf
- file example.otf
Windows (PowerShell)
- Format-Hex -Path example.otf -Count 16
Python
- open("example.otf","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the OpenType file signature?
OpenType files start with the hex bytes 4F 54 54 4F (OTTO in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.otf on Linux or macOS, or Format-Hex -Path example.otf -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the OpenType signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine OpenType file has these exact bytes; a file with the wrong bytes is not really OpenType, whatever its name says.