Reference

StuffIt (.sit/.sitx)

StuffIt (.sit, later .sitx) is a compressed archive format that was the standard way to pack files on classic Mac OS. It bundled and shrank files much like ZIP, and is now legacy, replaced by built-in ZIP support on the Mac.

Files & formatsmacOS

StuffIt (.sit/.sitx)

Also known as: StuffIt, .sit file, .sitx file, Mac StuffIt archive

Classic Mac OS standard compression format
Preserved Mac resource forks; .sitx added stronger compression
Legacy; opened by The Unarchiver or StuffIt Expander

The classic Mac archive

For years, StuffIt was the default compression format on the Macintosh, preserving Mac-specific file details (like resource forks) that early ZIP tools did not. The newer .sitx format added stronger compression.

Once macOS gained native ZIP support, StuffIt faded, but old downloads and backups still contain .sit and .sitx files.

Opening StuffIt files

The Unarchiver and the StuffIt Expander app open .sit and .sitx archives. After extracting, re-compress to ZIP for a format every modern system handles.

Related terms

Keep reading the reference.

Files & formatsZIP archiveA ZIP archive bundles one or more files into a single compressed .zip file that is smaller and easier to share. Unzipping it creates a second, full-size copy of the contents — so an archive and its extracted files can both take up space at once.General Files & formatsArchive fileAn archive file bundles many files and folders into one container — for example a ZIP, RAR, or 7z. Most archives also compress their contents to save space, but some (like plain TAR) only bundle without shrinking anything.General Storage conceptsCompression (lossy vs lossless)Compression shrinks a file by encoding its data more efficiently. Lossless compression rebuilds the original exactly, while lossy compression permanently discards detail the eye or ear is least likely to notice in exchange for much smaller files.General