A file with .123 extension is a spreadsheet file created with Lotus 1-2-3, a spreadsheet software by IBM Lotus SmartSuite. It had been in use for quite good times before it was surpassed by Microsoft Excel application in early 1990s. Data in a Lotus spreadsheet file used to be organized in rows and columns, similar to current Excel files, and different computations functions could be applied to it. Several different versions of Lotus spreadsheet file formats kept evolving with different extensions such as .wks, .wk1, .wk2, .wk3, and .wk4, with each varying numerical increment in extension consisting of modifications. Lotus was officially discontinued in 2013.
Lotus 123 File Format - More Information
Lotus files are binary files and can not be read or interpreted if opened in raw form. Its file format specifications are available as archives for reference at following links: