Line Sub-Structure in Human Design refers to the further subdivision of each of the 6 lines within a hexagram into 6 additional sub-positions, creating a total of 384 sub-positions across all 64 hexagrams (64 × 6 lines × 6 sub-positions = 384). These sub-positions are designated as Color (6 colors per line), and each Color is further divided into Tone and Base. This sub-structure is the foundation of Variable (PHS — Primary Health System), Ra Uru Hu's most advanced layer of Human Design mechanics. In Variable, the Color and Tone of the Personality Sun/Earth and Design Sun/Earth activations determine the individual's dietary regimen, cognitive style (digestion), environment, perspective, motivation, and sense of direction. The Line Sub-Structure is not visible in the standard BodyGraph display but is calculated from the precise degree of each planetary activation.