FF1 Linear Musings

When Does Each Numbered FF Let You Open World:
1 arguably doesn't, because you have to blunder into everything because NES.
2 was pretty linear almost the entire time; the plot basically holds you close.
3 was about the time you get the airship/sub ship, and only a couple extra things you don't even need (like, a couple summons that are nigh-worthless in the DS and later versions by the time you get them)
4 is after you lose cid to seal the underground, or after getting the falcon lava-proofed, because you can start getting summons (and whack Yang with a pan).
5 is merged world after the first tablet, because party (and further more after 2 tablets so you can get the airship after Fork Tower).
6 is after getting airship WoR; there's a reason CES is a possible (but butts) way to beat the game.
7 is, realistically, the first that basically starts and goes, "here's the open world, have at it" (clear midgar, and the  rails become far less defined and more "if you want to plot, go here to continue")
8 is after Garden is mobile to start with, but mostly post-space with the ragnarok.
9 has side stuff available in pieces early too...
10 is once you get to command the airship (so, post Bahamut)...
12 is where I stop counting because I haven't beaten any more starting there.