4. GUILLELMUS DURANDUS (1237–1296). Rationale divinorum officiorum. Printed on vellum. [Mainz]: Johann Fust and Peter Schoeffer, 6 October 1459.

The most important medieval treatise on Christian liturgy, the Rationale divinorum officiorum is Bridwell Library’s earliest complete printed book. The library’s collection includes several typographic specimens of earlier date, but these are fragments: vellum and paper leaves from three different Gutenberg Bibles (c. 1455), a single vellum leaf from Fust and Schoeffer’s second Psalter (29 August 1459), and a vellum leaf from the unsigned “36-Line Bible” (Bamberg? c. 1458-61). The fourth or fifth substantial book produced on a press, the Durandus was the first printing of any major work by a post-biblical author, and the first book printed in a smaller type designed for solitary reading and study.

Like the impressive Psalters published by Fust and Schoeffer in 1457 and 1459, the Rationale divinorum officiorum features large red and blue initials printed from two interlocking pieces of metal – one for the letter and one for the surrounding decoration. The printers also supplied the headings and small chapter initials in red ink. Although the results were beautiful, this laborious multicolored presswork soon was dismissed as an unnecessary extravagance. Later printers generally used only black ink, leaving it up to the buyer to arrange for any decoration.