The three I would pick are: 1. 1980s – 1990s Mainframe to PC 2. 1990s – 2000s PC to Web 3. 2000s – 2010s Web to Mobile In each of these transitions the soft

The three I would pick are: 1. 1980s – 1990s Mainframe to PC 2. 1990s – 2000s PC to Web 3. 2000s – 2010s Web to Mobile In each of these transitions the software written for the Old Way was incompatible with the New Way. (There have also been shifts in data storage and network protocols that others might point to as major transitions.)