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In page Quill:

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There is a small but significant difference between a pen knife and a quill knife, in that the quill knife has a blade that is flat on one side and convex on the other which facilitates the round cuts required to shape a quill.[citation needed] A "pen" knife by contrast has two flat sides. This distinction is not recognised by modern traders, dealers or collectors, who define a quill knife as any small knife with a fixed or hinged blade, including such items as ornamental fruit knives.[citation needed]