The Letters

[Pagliacci he/him]

Sorry if there is some obvious answer I'm missing but in regards to the letters that appear in Oathbringer between Hoid, Frost and Harmony, how are the letters being exchanged? 

[Weltall]

Brandon has RAFO'd the specifics but we know that there are people who travel between worlds who can hand-carry at least some of the letters.

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Overlord Jebus [PENDING REVIEW]

Cosmere postal service. Can you tell us anything about this? How many people are involved?

Brandon Sanderson [PENDING REVIEW]

It is more UPS than USPS, if that makes sense. Meaning it is enterprising individuals who are traveling and trading between planets. Some places are easier to deliver to than others.

Footnote: UPS is United Parcel Service (a package delivery company) and USPS is the US Postal Service.
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Questioner [PENDING REVIEW]

In the Stormlight Archive, there are letters in the epigraphs. With those letters, is there some sort of Shardic postmaster? Who's delivering these letters?

Brandon Sanderson [PENDING REVIEW]

I've been asked this before, actually. I wasn't asked it as early as I thought. I only started getting this one kind of recently. And my answer is RAFO. because, in part, it depends on the two planets you're talking about.

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We know that Nalthis is a worldhopper-friendly world and there's been a fair amount of travel to and from that world so I imagine getting a letter to Endowment is as easy as finding the next person heading in that direction and handing it off to them. Not too hard to get letters to Scadrial by Era 2 also I'd imagine. How the letters Hoid and Frost exchange get to each party on the other hand is anyone's guess since that requires knowing how to access Yolen and at least as late as MB Era 1 even Khriss didn't know how to do that.

[earthexile]

It could always be some kind of Spanreed situation, where there is a central hub that one can exchange messages with, using Invested technology.