Zenodotus created the first cataloging system wherein scrolls were sorted by subject, then type (primary historical source, secondary literary source, maps, new literature), then author.

Later, when the collection grew too large, the system was recast by Callimachus, who probably wrote the first library bibliography, the Pinakes. He subdivided the scrolls in the library into more genres - rhetoric, law, epic, tragedy, comedy, lyric poetry, history, medicine, mathematics, natural science, and miscellanies. The Pinakes listed the works in alphabetical order and contained the first instance of library metadata, notes on the author, the manuscript etc.

Source: Phillips, Heather A., "The Great Library of Alexandria?". Library Philosophy and Practice, August 2010

The solution to this problem was the creation of a principle according to which the holdings of the Great Library would be ordered. Zenodotus, the Great Library's first librarian, introduced a rudimentary organization system whereby texts were assigned to different rooms based on their subject matter.[165] Zenodotus first inventoried the Library�s holdings,[166] which he then organized into three major categories. The first category included history books, edited and standardized literary works, and new works of Ptolemaic literature. The second included holdings used for comparison and in the creation of the standardized works mentioned above. Included in this category were also letters and maps. The third group comprised original writings in foreign languages, many of which had been translated into Greek, and which, in translation were included in the first group.[167] Within each of these divisions, Zenodotus organized the works alphabetically by the first letter of the name of their author.[168] The principle of alphabetic organization, so unremarkable in modern days, was introduced by Zenodotus.[169]

In addition, library staff under Zenodotus attached a small dangling tag to the end of each scroll,[170] which contained information on each work�s author, title, and subject[171] so that materials could be easily returned to the area in which they had been classified, but also so that library users did not have to unroll each scroll in order to see what it contained.[172] Obvious and unimpressive though it may seem to those accustomed to modern libraries, this was the first recorded use of metadata, a landmark in library history.

Zenodotus� �subject/room� and alphabetization methods were a beginning, but as the collection grew, they became less and less effective. A more efficient system of organization was needed. So while Zenodotus made a good start at organizing the Great Library, in order to complete it, �one needed a scholar of encyclopedic knowledge and erudition as well as of infinite energy. Such a person was found in Callimachus.[173] Zenodotus� methods overlaid an ordering principle on the entire collection of the Great Library. While Callimachus did not deal with the entire collection, his work imposed a more specific order on the first, most heavily used and largest of Zenodotus� divisions. Callimachus divided this collection according to what were generally agreed at the time to be the main realms of literature.[174] Then within each of these divisions, he shelved all the authors in alphabetical order by the first letter of their name under their genres. Certain authors, of course, would be located under multiple genres. (Zenodotus had not physically placed them with such specificity).[175]

This took care of the shelving principle, but Callimachus went a step further. As a finding aid, Callimachus produced the pinakes,�[176] or "Tables of Persons Eminent in Every Branch of Learning Together With a List of Their Writings"[177] While the entire one hundred twenty scrolls[178] of the pinakes have not survived to this day, the pieces of it that have survived allow scholars to study it.[179] This was one of the first known documents that lists, identifies, and categorizes a library�s holdings.[180] Within the pinakes, Callimachus listed works alphabetically by author and genre. He did what modern librarians would call �adding metadata� -- writing a short biographical note on each author, which prefaced that author�s entry within his catalogue.[181] This helped avoid confusion in the works of authors with similar or identical names, but separating works of the original author and works of namesakes was often extremely difficult.[182] In addition, Callimachus noted the first words of the work, and the total number of lines in the document.[183] Later librarians were to make marginal notations in the pinakes, which provided even more information on the nature of the catalogued document.[184]

By consulting the pinakes, a library patron could find out if the library contained a work by a particular author, how it was categorized, and where it might be found.[185] The pinakes covered holdings in the Serapeum as well as the main Mouseion library.[186] It is important to note that Callimachus did not seem to have any models for his pinakes, and invented this system on his own.[187]