Think of it like a 0-indexed array: [a, b, c, d]
a is at position 0, b is at position 1…
This array has 4 elements despite the last element only being at index position 3.
A ‘2-story’ home would be a house with 2 different elevations:
[elevation a, elevation b]
If you want to refer to a specific floor, you need to use the index, which is 0/ground for elevation a, and 1/first floor for elevation b.
Im think it was designed with a clock in mind. That’s why it starts with north at the 12:00 position and goes clockwise. I learned it as ‘Never Eat Soggy Waffles’ but I’m sure there are a bunch of these.