The movable-frame hive rests on the discovery that bees leave a gap of roughly 6-9 mm unbuilt.
Lorenzo Langstroth's 1851 observation of bee space — that bees propolise gaps smaller than about 6 mm and build comb in gaps larger than about 9 mm — is the basis of the movable frame and of essentially all modern hive design. The account is well documented historically and the principle is directly observable in any hive.
- The World History of Beekeeping and Honey Hunting — Routledge (1999) · Historical text