Negative one overify

Funny the things that pop into your head from time to time.

"Negative one overify" was one that recently jumped up at me and I couldn't for the life of me remember what it was about - except that it was something to do with maths.

Then I realised, it was the way to convert a gradient to its perpendicular. In the plane at least. The little phrase was something I used back at school to describe the technique.

No clue why it appeared once more after being dormant for over twenty years - but I'm determined to find a use!

[Image credit: Michael Coghlan]