“A Genealogy of X” describes a method of “Chasing the City” by focusing on a singular spatial typology—the urban voids of Seoul—to construct a social narrative of public spaces distorted by personal desires. In architectural drawings, a big X demarcates a flat void as a hole in the floor that seldom continues beyond the single level immediately below the hole. A flat void is more conceptual than spatial. The X denotes a territorial gap rather than a spatial volume. Prominently marked with a big X in blueprints, a flat void quickly vanishes in section, or only a faint suggestion of its apparition remains in thin lines. The popularity of flat voids in Korean architecture cannot be traced back to a single source, nor can we identify its precise spatial or typological origin. But we recognize it when we see it. Subtracting, hollowing out, erasing, and puncturing solid volumes are more aggressive and sculptural acts of intervention, whereas leaving, maintaining, and composing empty spaces are more passive and sentimental. Developers and occupants plug many of these voids, often illegally, and the flat voids have suffered a long period of extortion and mutilation. Flat voids, understood as architectural manifestation of historical voids from wars, or emotional voids from oppression, can serve as a symbolic device that transcends its spatial physicality. Without repeating the weak aesthetic arguments of poetic emptiness, young Korean architects today have rediscovered and reconfigured flat voids in this more social, political, and symbolic context, and the X now spans more freely across various depths, widths, and heights.