The word 'substructure' is a transparent compound formed within English by combining the Latin prefix 'sub-' with the noun 'structure'. The prefix 'sub-', from Latin, consistently conveys the sense of being underneath or secondary. The core noun 'structure' originates from the Latin struere, "to pile up or build," which itself derives from the Proto-Indo-European root stereu-, "to spread, extend, or pile up." Therefore, the literal meaning "an underlying build" or "a foundation that is built beneath" maps directly and logically to its modern abstract sense of any supporting framework or base layer upon which a larger system or edifice depends.

Structure: sub (under) + structure (framework) - sub: A Latin preposition and prefix meaning "under, below, beneath." - structure: From Latin structura, meaning "a fitting together, building, arrangement." The root stru- means "to pile, build, assemble."