The Constraint as Design Tool
The original Adventure (1976) allowed players to carry any number of items. The adventure game's gradual imposition of carrying limits transformed the inventory from a storage mechanism into a decision space: choosing what to carry, what to leave behind, and what to return for introduced a resource management dimension that the genre had not previously contained.
The Resident Evil (1996) inventory — a fixed grid of rectangular cells in which items occupied different numbers of cells depending on their size — made inventory management a spatial puzzle with tactical implications. Shotgun shells occupied two cells; the shotgun occupied six; a first-aid spray occupied two cells and might be needed at any moment. Players who managed their inventory efficiently had a material advantage over players who carried items without system. The inventory was not administrative; it was a gameplay layer as demanding as the combat or the puzzle-solving.
Weight, Encumbrance, and the Spatial Grid
RPG inventory systems introduced weight as the limiting resource, producing a management problem with continuous rather than discrete constraints. Ultima IV (1985) and its successors tracked item weight in pounds; carrying too much reduced movement speed. The weight system reflected the simulationist ambition of the Ultima series, but it created administrative burden without equivalent decision depth. Counting weight across dozens of item types was calculation, not play.
Diablo (1996) resolved the weight problem by using a spatial grid but eliminating weight tracking: items occupied a fixed number of grid cells regardless of their simulated mass. The spatial puzzle of arranging items efficiently was immediately comprehensible and tactilely satisfying in ways that weight-unit tracking was not. Diablo's inventory design influenced every action RPG that followed it; the spatial grid became the standard against which subsequent inventory interfaces were evaluated.