In 1975, when Fred Brooks published his essay on conceptual integrity and system design, computers were used only by specialists: applied mathematicians, statisticians, scientists, engineers, and processors of business data. These specialists had to spend a lot of time studying technical manuals before they could use their computers.

Nowadays almost everyone uses computers or smart phones. Hardly anyone spends more than a few minutes reading the technical manuals. How did that happen?

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Xerox PARC (Palo Alto Research Center) developed the desktop metaphor during the 1970s. Apple Computer used Xerox's ideas in the Macintosh computer of 1984. Apple has continued to develop those ideas, emphasizing consistency and conceptual integrity. Microsoft copied many of the Xerox/Apple ideas for its Windows operating system, and went on to develop a slightly different style of user interface. Similar user interfaces are now used by Linux and smart phones.

And so, today, we know what to expect when we begin to use a new software product. We expect to use a pointing device or touch screen. We expect pull-down or pop-up menus to appear when we click icons, buttons, or links, and we expect those menus to have something to do with what we clicked.

With computer software, we expect to highlight things by holding down a button while dragging the cursor across those things. If we use the copy, cut, paste, or delete commands, we expect the highlighted things will be copied, cut, replaced, or deleted.

Depending on our machine, we may even expect certain keyboard shortcuts to work pretty much the same in every application program: command-C, command-X, command-V. (If your keyboard doesn't have a command key, chances are good that control-C, control-X, and control-V will work instead.)

Those expectations arose from the conceptual integrity of successful user interfaces.