Spanreeds look like ordinary writing reeds, except that each has a small infused ruby affixed to it. The other part of the same ruby is attached to its pair. This way, when someone writes with one of the reeds in the pair, the conjoined reed mimics its exact movements.
Request for communication, acknowledgment, and indicating whose turn it is to write is done by turning the spanreed gemstone to correct setting. When a request for communication is made, it causes the conjoined Spanreed to pulse slowly.
Spanreeds are one of the most common forms of pairing fabrials. Highprince Dalinar Kholin had five spanreeds in his study, one of them conjoined with Jasnah's. Shallan Davar and her brother Nan Balat are also known to have a pair.
During a spanreed conversation, Tyn discovers that Shallan is, in fact, Jasnah Kholin's ward, while - at the same time - Shallan discovers that it was Tyn who sent assassins after Jasnah. Between them, a fight ensues and Shallan winds up killing Tyn with her Shardblade.
When asked whether he had any specific inspirations spanreeds, Brandon replied with the following:
|“|| Writerly inspiration for spanreeds is me acknowledging that I wanted to have a society that acted more like a post-Industrial Revolution society (or very close to it) than a Medieval society. And there's lots of ways to do this. Fantasy worlds do not have to progress socially the same way that we progressed. A lot of people want to tie technology to social progression, which you don't have to do. You don't necessarily have to say people from the Industrial Revolution in our world acted this way; therefore people in this world ... . You just don't have to do that.
Real-world inspiration, if there is one, is an auto-pen. Where authors can have a little machine sign books for them; it moves on its own. I've never used one, but politicians use them quite a bit. When you get that hand-signed letter when you've donated whatever to whatever political party. That hand-signed letter was probably machine-signed with a real pen, rather than hand-signed by the individual.