For years, most operating systems maintained two lists of programs. One listed unopened programs until you needed them, like the Start menu (Windows) or the Launcher (Mac OS 9). The other kept track of which programs were open at the moment for easy switching, like the taskbar (Windows) or the Application menu (Mac OS 9).

In OS X, Apple combined both functions into a single strip of icons called the Dock. (And soon thereafter, Microsoft adopted the idea for Windows 7. But that’s another book.)