Libraries are among the most durable of human institutions. The great library of Alexandria, founded in the third century BCE, became a byword for civilisation within a few decades and was still famous centuries after its physical destruction. Medieval monastic libraries preserved classical texts through periods of political upheaval. Modern national libraries - the British Library, the Library of Congress, the Bibliotheque Nationale - house tens of millions of volumes and receive visitors from every country. What all of these have in common is not only collections of books but a set of institutional practices that govern how books are catalogued, lent, preserved and read.

The first library in anything resembling a modern sense appeared in Mesopotamia, where clay tablets storing legal, religious and mathematical texts were grouped and labelled for retrieval. The library at Nineveh, assembled by the Assyrian king Ashurbanipal in the seventh century BCE, contained more than thirty thousand tablets and was arranged by subject. What made it a library, rather than a heap of tablets, was the index: an accompanying set of documents that told a reader where to look. Indexing has remained the defining institutional act of library work ever since.

Alexandria extended this approach on an unprecedented scale. Its librarians attempted to assemble copies of every text in Greek, and some in other languages; its first chief librarian, Zenodotus, introduced the division of works into books and lines that is still used for ancient texts. Subsequent librarians, including Callimachus and Eratosthenes, developed the catalogue into a systematic work of reference. The library eventually declined through a combination of political upheaval, reduced royal patronage and a notorious fire during civil disturbances, but it set an example that later institutions drew on.

Medieval European libraries were smaller but methodical. Monastic scriptoria copied books by hand, and the resulting manuscripts were chained to the shelves of the monastery's library to prevent theft. Access was restricted to members of the monastic community and occasional visitors of suitable standing. Historian Dr. Henrik Fabricius has described these libraries as 'the quiet scaffolding on which Europe's literate culture stood': narrow in ambition compared to Alexandria, but effective at what they set out to do, which was to preserve particular texts across many generations.

The invention of printing, around 1450, changed the economics of libraries. Books became cheaper, private collections grew, and public institutions found that they could stock far more titles than before. The Bodleian Library in Oxford, founded in 1602, and the British Museum Library, founded in 1753, were among the earliest national-scale projects built around printed books. Both adopted legal deposit arrangements, under which publishers had to send a copy of every new book to the library, a practice that still underpins many national libraries today.

The twentieth century introduced two further transformations. The first was the rise of public libraries available free to all residents of a town or city. Andrew Carnegie's funding of more than twenty-five hundred such libraries between 1883 and 1929 was particularly influential; others followed in Europe and elsewhere. The second was mechanised cataloguing, starting with card catalogues in the late nineteenth century and continuing with electronic systems that now dominate library work. Dr. Fabricius has argued that electronic catalogues have done more to change library work since 1970 than any other development.

Digital collections have raised newer questions. National libraries now receive huge quantities of material born digital - online publications, websites, scholarly datasets - whose preservation requires technical infrastructure rather than simply shelf space. The British Library's UK Web Archive stores periodically refreshed copies of websites considered of cultural value, a task that did not exist before the 1990s. Ownership is also complicated: commercial databases that libraries subscribe to may be withdrawn if a contract lapses, leaving readers without the means to consult material they could previously access. Dr. Fabricius warns that this shift risks hollowing out the older library idea, which assumed that a book, once acquired, was in the library's permanent care.

The future of the library is not in doubt, but its form is changing. The oldest institutional practice, indexing, is now applied to items that have no physical presence; the oldest function, preservation, now includes software that may be obsolete in a decade. What has not changed is the basic promise: that a community pools its access to knowledge through a common institution, so that any reader, not only a wealthy one, can use it. That promise, Dr. Fabricius suggests, is worth protecting even as the means of keeping it alter.