Are there many other people confused like me about whether the top/bottom margins are horizontal or vertical, and vice versa for the left/right margins?
My inital thought was that the lesson is wrong in calling the top/bottom margins vertical, because I see these margins as running horizontally across the page, and the left/right margins as running up/down the page vertically…
I was so sure I was right, that I’ve sent an error report. HOWEVER, having just read this exercise about Margin Collapse, I can see that it clearly calls the collapsing margins vertical. Surely, it can’t be wrong throughout the lesson… 
Anyway, I checked Wikipedia https://en.wikipedia.org/wiki/Margin_(typography), and saw that top/bottom margins are measured vertically (even though they run horizontally) and vice versa for left/right margins!.. 
So, I was feeling guilty (and pretty sheepish) about my error report submission 
, when I then looked closely at the FAQ post for this exercise called… wait for it… What is the reason behind collapsing on horizontal margins and not vertical… 
…spot where I’m going with this? 
@axelkaban, are you confirming that my inital feeling was right (and the lesson misleading)? Or, are you just as confused as I am? 
Having written this, I’m now most definitely feeling like a collapsed margin myself!.. I’m not sure whether I’m vertical or horizontal… but definitely collapsed! 