The Constraint as Design Tool
Even the original Colossal Cave Adventure (1976) capped how many items the player could carry, and the adventure game's 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 — six or eight slots, with every item, however small, taking one — made carrying capacity a constant tactical decision, and Resident Evil 4 (2005) later turned it into a spatial grid puzzle. 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. Later Ultimas, from Ultima VI (1990) on, tracked item weight, so carrying too much stopped the Avatar picking anything else up. 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.