"Huh?" I said. "Please explain."
It turns out that during W.W. II, my grandmother and some other girls had a tight-knit group of friends in Lima, Ohio, where they lived and worked. After the war ended many of the women got married and moved out of town, so they decided to stay in touch by creating a letter circle. Each envelope contained a letter from every member of the group. When you receive it, you've seen none of the letters but your own from the last round, and so you read all the letters and then replace your letter with a new one and send it on. Each letter is really a missive to the entire group, and it saved both time and expense and sending individual letters.
Apparently they had been writing on a regular basis for 60 years, although my mom pointed out that the circle had gotten smaller. My mom asked my grandmother what she wanted to say in what would undoubtedly be her last letter to the group. She declined to dictate anything and merely said, "You know what you need to write." My mom composed a letter letting them know their friend was not long for this world, and that it was wonderful that their letters arrived when they did and could be shared.
I am impressed and fascinated by this. I had never heard of a round-robin letter before. An unbroken circle of communication and friendship, 60 years and running.