Complex sentence: Quill pens were the instrument of choice during the medieval era due to their compatibility with parchment and vellum, prior to this the reed pen had been used, but a finer letter was achieved on animal skin using a cured quill.
Split into simpler sentences: Quill pens were the instrument of choice during the medieval era due to their compatibility with parchment and vellum. Prior to this the reed pen had been used, but a finer letter was achieved on animal skin using a cured quill.