Fermin, B.; Kastelein, D. (2013). Het Zutphense Kwadrant. Archeologisch onderzoek in de gracht van de ringwalburg op de Houtmarkt te Zutphen [The Zutphen Quadrant. Archaeological research in the moat of the ringwalburg on the Houtmarkt in Zutphen] (in Dutch). Zutphen: Zutphense Archaeological Publications 80. doi:10.17026/dans-xyp-9pzw.
Silke Ackermann & John Cherry (1999). "Richard II, John Holland and Three Medieval Quadrants". Annals of Science. 56 (1): 3–23. doi:10.1080/000337999296508.