幽

Inner-left 幺, the indexing radical - pictograph of a single tiny silk filament, smaller than the doubled 糸. Carries the core meaning: 幽 was originally the silk thread so fine it disappeared into shadow, the dim and indistinct. Same family as 幻 illusion, 幼 young - all built on the diminutive thread.

Inner-right 幺 - the second filament, mirror-paired with its twin to redouble the barely-there-ness. Two threads concealed within an enclosure produce the dim recesses of 幽: secluded valleys, ghostly half-light, hidden depths (幽暗 gloomy). Reduplication intensifies the obscurity.

Outer 山 - the visual is split: leftmost downstroke opens the mountain shape, rightmost two strokes close it, with the two 幺 nested inside. Pictures threads hidden deep in mountain shadow - the literal source of 幽 'secluded, dark, faint.' The overlay structure gives the character its distinctive split silhouette.