Opus responsivum
Opus responsivum is a 1488 incunable by Petrus de Rivo, printed by Ludovicus de Ravescot at Louvain, Belgium, a scholarly treatise on the calendar and the date of Easter. The pictures are woodcut and printed diagrams: a tabular calendar with red and black lettering and columns of dates, a sky-and-horizon circle labelled Dies artificialis and Nox, and a framed hooded figure beneath a Gothic arch with an initial letter. A designer might take the crisp black-on-cream letterpress grid, the decorated initials and the simple circular diagram as a model for diagrammatic layout.
