But in this case you have 1,2,3,4,5 and start 1 end 4 selects 1,2,3. So it appears inclusive 1, exclusive on the 4 to me. If it were like some instances of substring where it is (index, count), I could get that, but it almost reads as "start right here, end right before this."

Actually, wait. Thought on your line numbers a bit and I'm thinking that points to each vertical line as a number (so in this garden's case, 6 per row). I just plugged it in as that and it follows. Just feels foreign to me as I'd do start/end 1 for just first cell, start 1 end 2 first two cells, etc. I'll get accustomed to it though. I wonder if there are other scenarios that followed the same logic in CSS that I just didn't attempt to really grok. Thanks for pointing it out.

I found this a strange decision in the Grid standard design. Surely most people* would think more in terms of "cells" that "cell-dividing-lines". Then again, compared to the design decisions in much of the rest of CSS including every other standard layout mechanism this is a fairly small gripe, overall CSS Grid is a big improvement. And when using the excellent "grid-template-areas" (compared to ASCII art elsewhere in this thread) you don't really specify start and end points anyway.

* By people, I mean designers (used to bootstrap-style grids), coders (used to arrays) and everyone else (used to spreadsheets).