Chapter 2 called it slope. Road signs call it grade: $G = \dfrac{100\,\Delta h}{L}$ — G equals one hundred times the rise, over the run, where $\Delta h$ (delta-h) is the change in height, $L$ is the LEVEL run in the same unit, and $G$ is the grade itself, in percent. An 8% hill climbs 8 m for every 100 m of LEVEL run. Earthworks crews wear a third costume: an n:1 slope, n metres across for each 1 down, so $G = \dfrac{100}{n}$ — a gentle 4:1 bank is a 25% grade.

And here is this chapter’s handshake: rise over run is exactly $\tan\theta$. So $G = 100\tan\theta$, and $\theta = \tan^{-1}\!\left(\dfrac{G}{100}\right)$ turns any grade into its angle. A 100% grade is not a wall — it is $\tan^{-1}(1) = 45^\circ$. Say that at dinner and watch somebody argue. You will win.