空kōng

Modern simplified form (簡體). 空 (kōng, “empty, sky”) is identical in simplified and traditional — the top is the radical 穴 (xué, “cave”), and the bottom is 工 (gōng, “work”), together suggesting a hollow or opening. It serves as the radical in 控 (control), 腔 (cavity), and 穷 (poor), and its meaning of “nothingness” is reflected in phrases like 空气 (air) and 空前 (unprecedented).