1935 One evening, Bioy Casares mentions to Borges a land called Uqbar about which he has been reading in a volume of the Anglo-American Cyclopedia. An edition happens to be in the suburban house they have temporarily rented, but after consulting it they find that there is no entry for Uqbar. Bioy Casares insists that he has read about it in the Cyclopedia and some days later he produces a volume which looks similar to the one they had consulted, except that it has a dozen more pages, where Uqbar is registered as an entry. The information given in it is vague and names Tlön as a mythical region of Uqbar.
1937 or 38 Borges finds the eleventh volume of A First Encyclopedia of Tlön which was sent to Herbert Ashe, a man that he had met several times in a suburban hotel. This volume, whose first page is stamped with an oval bearing the inscription 'Orbis Tertius', has precious information about Tlön, much of which Borges presents as philosophical narrative situations in the story.
1941 A letter is discovered from Gunnar Erfkord to Herbert Ashe, in which the enigma over Tlön is partially resolved. At the beginning of the seventeenth century a secret society conceived of the task of inventing a land. Each member of the sect had to elect a follower in this, in principle, endless or almost endless enterprise. After two hundred years of silent, or secret or interrupted operations, the society reappears in America. In 1824, one of its members recruits a millionaire who is enthusiastic about the project and proposes the more ambitious plan of inventing not just a land but a planet. At last, in 1914, the society is able to publish the final volume of the First Encyclopedia of Tlön, that would provide the basis for a revision of all the work done, but this time it is written in one of the languages of Tlön. This revision of an imaginary world is given the name of Orbis tertius (the Third World: a phrase which had none of its current resonances when Borges wrote his story). The volume that was sent to Herbert Ashe and was found by Borges in 1937 belongs to this version of the Cyclopedia.
1942 Very strange objects, from Tlön, begin to appear, of all places, in Argentina: they are very heavy and Borges and one of his friends find one of them in a remote rural saloon in the pampas.
1944 A journalist in Nashville discovers the forty volumes of the First Cyclopedia of Tlön. Borges writes a post-script which ventures the hypothesis that all countries and languages are bound to disappear and that the real world will become Tlön.
These are the 'facts' of the plot, that could be seen as the external history of the discovery of Uqbar. I have reordered and simplified Borges's much more complex sequence of events. Borges orders this material through two of his favourite devices: false attributions to a mixture of existing and invented texts, and the introduction of many of his real life friends. Thus, the limits between what really happened, what could have happened and what could never happen are interwoven through a method of verisimilitude which supports an invention with the name of a real existing person, and attributes to books whose nature is ambiguous (they could exist, they appear to be existing books) the origin of a fabulous situation or a necessary quotation. Needless to say, this method of attribution and verisimilitude questions the status of reality, and also points to the permeable nature of fiction which longs to grasp something that is always escaping it.