The Apple II, also invented by Wozniak, was introduced on April 16, 1977, at the first West Coast Computer Faire. It differed from its major rivals, the TRS-80 and Commodore PET, because of its character cell-based color graphics and open architecture. While early Apple II models used ordinary cassette tapes as storage devices, they were superseded by the introduction of a 5 1/4 inch floppy disk drive and interface called the Disk II.[26] The Apple II was chosen to be the desktop platform for the first "killer app" of the business world: VisiCalc, a spreadsheet program.[27] VisiCalc created a business market for the Apple II and gave home users an additional reason to buy an Apple II: compatibility with the office.[27] Before VisiCalc, Apple had been a distant third place competitor to Commodore and Tandy.[28][29]