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.