The early-90s Windows 3.11 operating system offered a graphical user interface that was a breakthrough for me. It was, in fact, my first GUI. I’d been using command-line, error-prone MS-DOS for two or three years before that, and it was a delight to suddenly be able to maximize screens, switch programs, and point around with a mouse, after living with the syntactically regimented MS-DOS. I bought a new laptop then to coddle this exciting visual operating system, junking my aging, redundant command-line clamshell. 
