This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

As one of the earliest centers of book-printing in Europe (see above: History), Strasbourg for a long time held a large number of incunabula—documents printed before 1500—in her library as one of her most precious heritages. After the total destruction of this institution in 1870, however, a new collection had to be reassembled from scratch. Today, Strasbourg's different public and institutional libraries again display a sizable total number of incunabula, distributed as follows: Bibliothèque nationale et universitaire, ca. 2 098 Médiathèque de la ville et de la communauté urbaine de Strasbourg, 394 Bibliothèque du Grand Séminaire, 238 Médiathèque protestante, 94 and Bibliothèque alsatique du Crédit Mutuel, 5.
What are incunabula's?