I'm pretty sure someone this weekend blogged about how in the 1st edition DMG section on minis Gygax makes it clear that 3 figures should be able to fit abreast in a 10' corridor. I have been considering using one square = 3.33 feet for a new tactical display I've been toying with building but I usually use 2 figures per rank in 10' wide corridors. But if someone really wanted to try for 3 I wouldn't object. It would just be more cramped and possibly more fumble prone. (You know what I need? A chart full of non-combat fumbles. Torches dropped for no good reason. Stumbles leading to noses broken on flagstones. Spilled backpacks. That sort of thing.)
Anyway, I just wanted to point out that to use the AD&D rules as written to determine the number of people in a rank you also have to look in the Players Handbook. I'm thinking of the Weapon vs. Armor Type chart here and the column labeled "Space Required". You can only fit one character wielding a Bec De Corbin, Footman’s Flail or Two-Handed Sword in a 10' wide space or two characters using Bardiches, Halberds, Lucern Hammers or Morning Stars.
Sure, you could squeeze a couple of spearmen next to that maniac with the two handed sword, but you're going to feel like a chump if he accidentally whacks you with that big clumsy blade.
I should point out how this situation is just one of many rules where half the info you need is in the DMG and the other half is in the PHB, like how both books have spell sections or to roll to-hit you need the DMG but to roll damage you need the PHB. Or how the crappy psionics rules in the back of the Players Handbook requires the crappy psionics rules in the DMG to function crappily.
Also worth noting here are the marching order rules in Empire of the Petal Throne: three characters abreast is standard just like in the DMG; four may squeeze in if none of them are fighters; only two may fight side-by-side if one wields a two-handed weapon, except for a two-handed sword. If you wield a two-handed sword you take up the whole dang rank by yourself.
Dungeon/Ruin Environmental Hazards
1 hour ago