Dos-á-dos

Dos-á-dos is a type of folding used for book binding. In practice, it refers to binding two books together by combining the back covers of both books into one part. "Dos-á-dos" is a French phrase that translates to "back-to-back," literally meaning how the books are bound. This method of binding was useful for when two texts were needed at the same time, or heavily correlated with one another. As an example, the New Testament and the Psalter were often taught together at the time of the dos-á-dos's invention.