I'm kind of borrowing this word from Bookwitch's post, if not outright stealing. Haven't read Bookwitch, even though I have her blog right there in the blog roll? You should check her out sometime. You don't have to be a fan of children's literature or crime fiction to enjoy her original mind. If she didn't state it boldly, you would never know that she was Swedish, living in England for many years and now Scotland. She is a much better writer in English than she thinks she is, or than most of us native speakers actually are.
In her latest blog post, she admitted feeling a bit perplexed about the precise meaning of "perpendicular". She knew vaguely, but vaguely wasn't good enough when it came to giving directions at a crucial moment. She straightened it out in time, realizing that it meant "at right angles", but a regular commenter there said that she had always understood it to mean "vertical". As in candles on a cake. Both vertical and perpendicular do make sense in that context.
Anyway, I decided to get a little deeper into this. It turns out that the commenter who had assumed verticality has a reason for thinking that. According to the Online Etymology Dictionary. "Perpendicular" comes to us via Old French (perpendiculer) from the Latin perpendicularis--"vertical, as a plumb line". This in turn came from the word perpendiculum, which means "a plumb line". Think about it. What is more perpendicular to the ground than a plumb line?
The verb that perpendiculum comes from is perpendere, which means "to balance carefully". "Per-" is thoroughly, pendere means "to weigh or to hang".
"Weigh" keeps coming up. We'll get to it.