123 file extension is native to Lotus 1-2-3, a spreadsheet program. Lotus 1-2-3 was for quite a long time the standard spreadsheet application on MS-DOS operating systems.

123 files structure is typical for spreadsheet documents. Document is divided into rows and colums with cells. Cells can hold data inputed manually or by calculated using so-called formulas. Single 123 spreadsheet document can contain various types of data – calculations, tables, lists, charts, etc. 123 format supports many basic database operations.