This artwork is mysterious to me! It was featured on The Toast with the joke that he is writing with two pens. So what is the thing in his left hand? It looks a bit like a knife, and I know scraping mistakes off your manuscript with a knife was done, but were people actually in the habit of writing with the knife held at the ready in the left hand?
Manuscript images like these can be confusing in their relationship to reality.
Yes, this is a typical medieval manuscript image of scribe (or illuminator for that matter) with both quill and knife in hand. But...the knife would have been used for a number of purposes beyond scraping vellum corrections, including sharpening/adjusting the quill and possibly useful for flattening vellum surfaces where there are imperfections; these would have been done with knife simultaneously in the non-writing hand.
However, the key to our 'mistrust' of this picture's absolute verisimilitude to manuscript working methods is that scribe is working on a fully-bound book: scribes/illuminators worked in loose sheets that were assembled and bound after illumination. So, we can't take this picture for absolute indication of real working methods, instead it is communicating ideas through manuscript iconography.
More:
I don't know if this was ever the practice in Christian societies, but in both Jewish and Islamic cultures of the Middle Ages, there was a practice of "refreshing" the text of a sacred book by carefully, one letter at a time, scraping away the faded text, and instantly writing in a new letter in the identical footprint of the former letter. The idea here was to be able to "renew" faded manuscripts of holy books, without technically defacing or destroying the holy writ, which would have been unnacceptable.
