Siamese twins
The bookbindings above are as odd as they are rare. In fact, I encountered my first only a few days ago while browsing Folger Library’s image database of bookbindings. The binding is called “dos-à-dos” (back to back), a type almost exclusively produced in the 16th and 17th centuries. They are like Siamese twins in that they present two different entities joint at their backs: each part has one board for itself, while a third is shared between the two. Their contents show why this was done: you will often find two complementary devotional works in them, such as a prayerbook and a Psalter, or the Bible’s Old and New Testament. Reading the one text you can flip the “book” to consult the other. The last image above is a unification of no less than seven devotional works printed by the same printer (Feichtinger, Lintz, 1736-1737), showing that the constructions could also encompass much more than just two texts. In the 20th century this type of binding enjoyed a revival with the Double Ace books, which featured two short science fiction stories.
Pics: St Andrew’s University Library, Bib BS2085.C27 (top); Washington, Folger Shakespeare Library, STC 23811.2 (two pics), STC 2907 (broidery); Chetham’s Library, shelfmark unknown (editions from 1629, 1633); Ed. J. M. Feichtinger, Lintz, 1736-1737 (from this sales catalogue). Other examples from the Folger here. A nice one auctioned off at Christie’s here.