Lotus 1-2-3 is a spreadsheet program from Lotus Software (now part of IBM). It was the IBM PC's first "killer application"; its huge popularity in the mid-1980s contributed significantly to the success of the IBM PC in the corporate environment.
The Lotus Development Corporation was founded by Mitchell Kapor, a friend of the developers of VisiCalc. 1-2-3 was originally written by Jonathan Sachs, who had written two spreadsheet programs previously while working at Concentric Data Systems, Inc. To aid its growth, in the UK, and possibly elsewhere, Lotus 1-2-3 was the very first computer software to use television consumer advertising.
1-2-3 was released on January 26, 1983, started outselling then-most-popular VisiCalc the very same year, and for a number of years was the leading spreadsheet for the DOS operating system. Unlike Microsoft Multiplan, it stayed very close to the model of VisiCalc, including the "A1" letter and number cell notation, and slash-menu structure. It was free of notable bugs, and was very fast because it was programmed entirely in x86 assembly language and bypassed the slower DOS screen input/output functions in favor of writing directly to memory-mapped video display hardware.
This reliance on the specific hardware of the IBM PC led to 1-2-3 being utilized as one of the two litmus test applications for true 100% compatibility when PC clones started to appear in the early- to mid- 80s. 1-2-3 was used to test general application compatibility, with Microsoft Flight Simulator being used to test graphics compatibility. Because all of a spreadsheet needs to be resident in memory, it also drove the race to utilize more memory, and extended memory and expanded memory techniques were needed to overcome the DOS limit of 640KB to allow larger spreadsheets - this was so important that a memory used/remaining indicator was displayed on-screen.