The word 关键 is formed by combining two characters, each contributing a concrete, physical image that together create a powerful abstract concept. The first character, 关, originally depicted a bar or crossbeam used to secure a gate. The second, 键, referred to a metal bolt or latch. Thus, the compound visually fuses the idea of the barrier (关) and the mechanism (键) that locks or unlocks it. This combination metaphorically represents the essential, pivotal point that secures or controls a situation, the "linchpin" or "key" without which the whole system (like a gate) cannot function or be accessed.