When observing Madame Eglentyne’s white beautiful forehead, the narrator exclaims: “It was almoost a spanne brood, I trowe” (GP 155). What does it mean by the term “a spanne brood”? Is it “too much” for a forehead? Does it imply intelligence or “boldness”? Is it a sign of beauty or “stupidity”? Is it apposite for a religious woman to expose her forehead in public? In what way should a nun wear her wimple? By discussing these questions, this essay will inspect the misreading and misunderstanding of the Prioress caused by the ambiguous concept of the term “span.” This research proceeds in two aspects: 1) investigating the various definitions of the term “span” in the Middle Ages; 2) examining the portraits of courtly and religious ladies depicted in a number of medieval paintings and artworks. By analyzing and comparing the data, this essay intends to offer a more exact measuring of the Prioress’s forehead and will also propose that Madame Eglentyne’s broad forehead is simply a sign of beauty and that her beauty has little relevance to her piety.