How many college students today ever flip through trays of library catalogue cards? Some of them may never have used an actual tabbed file. But the tab as an information technology metaphor is everywhere in use. And whether our tabs are cardboard extensions or digital projections, they all date to an invention little more than a hundred years old. The original tab signaled an information storage revolution and helped enable everything from management consulting to electronic data processing.

The tab’s story begins in the Middle Ages, when the only cards were gambling paraphernalia. Starting in the late 14th century, scribes began to leave pieces of leather at the edges of manuscripts for ready reference. But with the introduction of page numbering in the Renaissance, they went out of fashion.

The modern tab was an improvement on a momentous 19th-century innovation, the index card. Libraries had previously listed their books in bound ledgers. During the French Revolution, authorities divided the nationalized collections of monasteries and aristocrats among public institutions, using the backs of playing cards to record data about each volume.

The idea of a randomly accessible, infinitely modifiable arrangement of data flowered first in the United States. Not that America had more books to organize. In 1820, the Göttingen University library in Germany already had 200,000 volumes; Harvard University had fewer than 118,000 books in 1861, when it became the first major library to use cards. The historian John Higham called the catalogue a “revolutionary” and characteristically American tool, which promoted specialization by grouping authorities together under topic headings and integrated the latest books rapidly—features we take for granted now.

It took decades to add tabs to cards. In 1876, Melvil Dewey, inventor of decimal classification, helped organize a company called the Library Bureau, which sold both cards and wooden cases. An aca­demic entrepreneur, Dewey was a perfectionist supplier. His cards were made to last, made from linen recycled from the shirt factories of Troy, NY. His card cabi­nets were so sturdy that I have found at least one set still in use, in excellent order. Dewey also standardized the dimension of the catalogue card, at three inches by five inches, or rather 75 millimeters by 125 millimeters. (He was a tireless advocate of the metric system.)

Even the Library Bureau did not offer a convenient way to separate groups of cards, apart from thin metal partitions that wrapped around them, or taller cards. The tab was the idea of a young man named James Newton Gunn (1867–1927), who started using file cards to achieve savings in cost accounting while working for a manufacturer of portable forges. After further experience as a railroad cashier, Gunn developed a new way to access the contents of a set of index cards, separating them with other cards distinguished by projections marked with letters of the alphabet, dates, or other information.

Gunn’s background in bookkeeping filled what Ronald S. Burt, the University of Chicago sociologist, has called a structural hole, a need best met by insights from unconnected disciplines. In 1896 he applied for a U.S. patent, which was granted as number 583,227 on May 25, 1897. By then, Gunn was working for the Library Bureau, to which he had sold the patent. It was to be a perfect match. The Bureau was becoming a leading supplier of corporate record-keeping equipment, offering “commercial grade” cards on wood-pulp stock.

The Library Bureau also produced some of the first modern filing cabinets, proudly exhibiting them at the World’s Columbian Exposition in Chicago in 1893. Files had once been stored horizontally on shelves. Now they could be organized with file folders for better visibility and quicker access. Tabs were as useful for separating papers as for organizing cards. Since business people were unfamiliar with the new technology, Library Bureau staff provided consulting services as well as equipment and supplies. By 1913, the company was advertising in the New York Times that it could supply a credit department with a 16-by-16-by-20-inch cabinet to “keep tab” on up to 14,000 customers. The Library Bureau also worked with Herman Hollerith, whose electrical punch card system later became the foundation of IBM.

James Newton Gunn went on to found one of the first consulting firms focusing on industrial engineering. He became an automotive and rubber industry executive. He helped found Harvard Business School and lectured at MIT, among other places. But the tab is his lasting legacy. And it is ubiquitous: in the dialogue boxes of Microsoft Windows and Mac OS X, at the bottom of Microsoft Excel spreadsheets, at the side of Adobe Acrobat documents, across the top of the Opera and Firefox Web browsers, and—even now—on manila file folders. We’ve kept tabs.