Correspondence - epistolary style
That is an awesome question!
The truth is, the choice of style came first - I wanted to write something that way, and then I had to figure out a story that it would fit.
I'd been on omegle, doing RPs, and I came across an epistolary prompt in your standard quasi-medieval setting, and... look, I know WAY too much about the Middle Ages for your standard quasi-medieval setting. I keep noticing things.
And what I noticed on this occasion, what kept bugging me was: They didn't write letters that way.
Some actual examples from the Middle Ages - taken from http://athenaeumhectoris.blogspot.com/2014/06/writing-medieval-letter.html - are
- To her dearest lord and father, Louis, by grace of God king of the French, M[arie] countess of Troyes, his beloved daughter, greetings and deepest love. (A letter from an unnamed man to Marie of France, undated.)
- To the queen of the French, abbot William. (A letter from Abbot William of St. Thomas of the Paraclete to Ingeborgof Denmark, 1195.
- Adelaide [Aelis] who was the wife of lord John of Avennes, defender of Holland and of Zeeland, to the castellan of Ath and all his sergeants, greetings. (A letter from Aleid of Holland to the castellan of Ath.)
Now, telling someone who's come up with a prompt 'you're doing it wrong' is a dick move, obviously. So if I wanted to write in this style, which I suddenly wanted to do, I needed to come up with a context to do it in. And I started wracking my brains, thinking: 'out of the characters I'm already familiar with, who would write letters this way?'.
And it suddenly occurred to me: Augusta Longbottom.
Augusta Longbottom would write letters this way.
She could write to Minerva McGonagall.
And literally everything else about Correspondence evolved from there.