Why?
Multiple reasons.
Then as now, writers started with rough drafts—but that’s a discussion for later (a good one).
It’s challenging to write neatly in a codex (the binding distorts the page, so the page isn’t flat).
Humans make errors; you don’t want to mess up the whole https://abs.twimg.com/emoji/v2/... draggable="false" alt="📖" title="Open book" aria-label="Emoji: Open book">
…

Another factor: ‘writing support’ (= stuff for writing on) was *Not Cheap*.

This is particularly true of the most widespread support for medieval books—namely parchment. Parchment is specially-treated animal skin.

(https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera">: Finished parchment sheets on the frames used in making it)

For a large book, one sheet of parchment = one life (https://abs.twimg.com/emoji/v2/... draggable="false" alt="🐄" title="Cow" aria-label="Emoji: Cow">/https://abs.twimg.com/emoji/v2/... draggable="false" alt="🐏" title="Ram" aria-label="Emoji: Ram">/https://abs.twimg.com/emoji/v2/... draggable="false" alt="🐐" title="Goat" aria-label="Emoji: Goat">).

To pick up a medieval parchment manuscript is to hold an entire herd in one hand.

https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera">1: A medieval manuscript taking an afternoon stroll.
https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera">2: Another medieval manuscript enjoying the early summer weather. https://abs.twimg.com/emoji/v2/... draggable="false" alt="😏" title="Smirking face" aria-label="Emoji: Smirking face">

That’s money, honey.

You aren’t going to slaughter 200 head of cattle, pay craftsmen for the WEEKS needed to turn each skin into parchment, bind all those *blank* sheets into a book, then stick it on a shelf until inspiration strikes.

[gif: “Dollar, dollar, bills y’all”]

Due to the foregoing (& other issues that we won’t get into here) medieval books were modular.
The fundamental unit of the medieval manuscript is the bifolium (plural: bifolia): a single sheet folded in half.

This [again, as a broad rule] is what scribes producing or contributing to books wrote upon.

Bifolia could be gathered together into ‘quires’ (https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera"> 1), then stacked in order (https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera"> 2), and sewn together and bound (https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera"> 3) to create the kind of book we see Brienne of Tarth writing in

(but note how the leaves below *aren’t blank*)

[https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera"> 2: @V_and_A]
[https://abs.twimg.com/emoji/v2/... draggable="false" alt="📷" title="Camera" aria-label="Emoji: Camera"> 3: @FitzMuseum_UK]

… or not. Many manuscripts were never finished, remaining in a state of ongoing or extended production—often deliberately (but that, too, is a discussion for another time).

Note that this subtopic is very complex and I’ve simplified radically in this section of the thread.

Nonetheless, as a medieval ‘scribe’, she would not be sitting down to write in a massive, bound, decorated (we’ll get to that https://abs.twimg.com/emoji/v2/... draggable="false" alt="🙃" title="Upside-down face" aria-label="Emoji: Upside-down face">) book.

A caveat: https://twitter.com/stephanielahey/status/1131048051336855552?s=21">https://twitter.com/stephanie...