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.