Urban form is never produced by a single rule, yet it is never free from rules. What planning governs most effectively is not the final shape of the city but the field within which shape can emerge. A limit on height, a minimum open-space ratio, a floor-area ceiling or a street-width requirement does not contain an architectural solution. Each creates a boundary condition. The built environment appears through the intersection of these conditions with property, finance, design practice, infrastructure and precedent. The relevant object is therefore neither regulation nor form in isolation, but feasible form: the changing set of spatial configurations that remain possible as rules accumulate and interact.

This shift displaces deterministic accounts of planning. Moroni’s distinction between teleocratic and nomocratic regulation clarifies why a rule cannot be read simply as a command toward a desired image. General rules can structure action without specifying an end-state, while discretionary planning can seek outcomes through case-specific steering. Both nevertheless alter the range of spatial possibilities. Regulation acts morphologically when it changes that range: when one option becomes impossible, another more advantageous, or a previously indifferent relation between footprint, height and access becomes decisive. Spacematrix gives feasible form a measurable topology. Density ceases to be a scalar and becomes a multidimensional configuration in which built intensity, coverage and network structure interact. Under the same nominal density, radically different fabrics remain available. This is more than a technical refinement. It means that regulation operates in a space of alternatives rather than along a single quantitative axis. A planning parameter does not merely add or subtract capacity; it reshapes the geometry of the solution field. The Nanjing evidence demonstrates that such effects can be observed historically. Plot indicators, building-density controls and block regulations leave traces in subdivision, massing and street-block structure, yet those traces remain mediated by developer response and broader urban transformation. Feasible form is therefore probabilistic rather than deterministic. Its analytical value lies precisely in the capacity to distinguish strong constraints from weak influence, and direct morphological effects from those routed through other institutional or economic mechanisms. For Socioplastics, feasible form offers an operational way to connect institutional language with spatial evidence. Regulations, plans, diagrams, massing studies and built outcomes can be archived as successive states of one constrained field. The archive no longer records only what was built; it can register what became possible, what was excluded and which combinations of rules produced each narrowing. This makes the regulatory environment observable as a material and epistemic infrastructure. Feasible form adds to Socioplastics a grammar of constrained possibility: a way of treating rules not as static texts but as active delimiters of spatial variation, and morphology not as a finished object but as the visible residue of successive institutional selections.