Someone finally asked.
After months of sending this weekly email, someone asked the question:
“What is a Plumbline, and why that name?”
Great question. Here’s the short version.
Technical definitions sound like this: "a plumb line is cord that is suspends a metal weight with a pointed tip on the bottom, and which points directly to the earth's center of gravity; it is used as a vertical reference line, perpendicular to the earth (it determines whether or not something is straight up and down)."