The Nuttall Encyclopedia
Name given to the rule laid down by Aristotle that a tragedy should be limited to one subject, to one place, and a single day.
Wood, James, ed. Entry for 'Unities, Three'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/u/unities-three.html. Frederick Warne & Co Ltd. London. 1900.