Sending Stone
Sending Stones are an arcane technology that allow for long distance communication. They come in pairs, with each stone often carved to match the other so the pairing is easily recognized. While someone holds one of the stones, they can cast the spell Sending at will, with the target being the person who holds the stone's twin.
Sending stone pairs can also be linked together by weaving a magically thread between nonpaired stones. This allows the message to pass through the linked stones and to all stones within the weave.
For example, if we have Sending Stone Pair 1 with Stone A and Stone a, Pair 2 with B and b, and Pair 3 with C and c, you could weave Stones a, b, and c together, at which point a, b, and c's only function is to pass a message on to the others. Now, if someone holding Stone A sends a message, both Stone B and C receive it at the same time.
When many pairs of Stones are directly woven together, this is call an open line. While an open line is easy to maintain, it is very unsafe and shouldn't be used unless by a small, trusted group of people.
Alternativly, most cities send their twined stones to a switchboard organization which connects a person to their intended recipient, allowing for one-on-one messages while still only needing one set. This is known as the Weave of Communication.