3 abreast in a 10' Corridor: Photographic Evidence from J E Holmes
Jeff Rients recent post jogged my memories of playing with old school 25mm minis and this plate from Dr Holmes' book "Fantasy Role Playing Games" seems to support it. Citadel and their 1 inch plastic bases have a lot to answer for.
From "Fantasy Role Playing Games" by J Eric Holmes MD (p177)
A tense moment. The cloaked figure of the thief (Ral Partha) crouches listening at the door as
the rest of the party (Ral Partha) led by the Elf mage (Dragon Tooth) guard the corridor intersection.