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What is a .123 file?

A .123 file is a Lotus 1-2-3 Worksheet (version 97). This one is a specialist format without an obvious everyday home. Confusingly, this extension has been used by 2 different formats over the years — they're all listed below.

This extension has been used by more than one format

2 different formats have claimed .123 over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .123 file

The program that created it is by far the most reliable way to open it.
On Windows, use a hex viewer or the free TrID tool to identify what the file really is.
On Mac or Linux, running `file yourfile` in the terminal identifies most formats instantly.

The registry records this extension against Lotus 1-2-3 Worksheet, Lotus 1-2-3 Worksheet, so software from that family is the most likely match.

How to convert a .123 file

Identify the file first, then look for an export or Save As option in whatever program can read it.

MIME type

When a .123 file travels over the web, servers label it with one of these MIME types:

application/vnd.lotus-1-2-3application/x-123

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .123.