Files that contain the .123 file extension are most commonly used by the Lotus 1-2-3 spreadsheet software application. The program was initially developed by Lotus, which was later acquired by IBM. The spreadsheet application was most popular in the mid 1980s. Over the years, the popularity of the Lotus 1-2-3 application was replaced by Microsoft's Excel.
The 123 files that are created by the Lotus 1-2-3 program contain spreadsheet data files that have been created within the software.
The .123 file extension is also used for circuit board designs that have been created by the PCB123 circuit board prototype tool.
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