Why end with 30?

At the bottom of each entry in this site’s blog, I use </30> to designated the end of the it.

In my own way, it’s a nod to the tradition of ending newspaper stories (and occasionally broadcast and magazine pieces) with - 30 -.

While no one’s sure why 30 was used as an end sign, there is no shortage of ideas. —Craig SailaWhy end with 30? (Living Can Kill You)

Somebody e-mailed me this question. In the back of my mind, I thought, “An obsolete typesetting symbol?” but I didn’t know.