Bottom 土 'earth' - the indexing radical, depicting the ground beneath the seated figures. Provides the surface they sit on. The compound is purely pictographic with no phonetic component, which is unusual: it is pure visual storytelling, two people on the dirt.
Top-left figure. Together with the second 人 to its right, two people face each other across a piece of ground. The doubled person sets up one of Chinese's most pictographic compound ideographs: a shared seat, the act of sitting together.
Top-right figure mirroring the first. The reduplicated 人 creates a vivid scene above the earth - two people seated on the ground together. 坐 is one of those rare compounds where the pictogrammatic logic survives intact in the modern form.
