I think that may also be the reason why medieval scribes wrote rather between than on the guidelines .
Indeed,
@Estefa &
@AnasaziWrites -- and also, visible guidelines can contribute to the design of a page. Scribes didn't always try to hide them. That's why they sometimes used the pricks on the verso side to draw lines to match the recto, rather than use the stylus indentation method or marks that could be rubbed off or minimized. The guidelines form neat borders around the text, and plenty of manuscripts leave them in.
It reminds me how every time I teach the canons of page construction, some of my students get so invested in the process that they choose to not erase the guidelines. They spend a lot of time redoing their work because they have to learn how to strive for accuracy, so by the time they get it right, they've put so much time into it to get the geometry right and the lines straight, they're very proud of their work. They want to show off those hard-earned guidelines.
And then, sometimes students don't want to ink the text onto the page that they've laid out, because they know that if they make a mistake (that they themselves find intolerable - I'm always telling them to live with their mistakes), and they want to start again, they'll have to do all the geometry and all of the pricking and ruling all over again (this is how many of them actually do learn to live with their mistakes).