From Technology Review‘s “Keeping Tabs“:
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. …
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 acaÂÂdemic 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 cabiÂÂnets 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.) …
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. …
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. …
But the tab is [Gunn’s] 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, andâ€â€even nowâ€â€on manila file folders. We’ve kept tabs.