The idiom 一清二楚 (yī qīng èr chǔ) is formed through a classic Chinese rhetorical structure of numerical parallelism, where the numbers "一" (one) and "二" (two) are paired with near-synonyms "清" (clear) and "楚" (clear/distinct) to create a balanced, rhythmic four-character phrase. This structure reinforces and intensifies the core meaning, "perfectly clear" or "crystal clear", as the two halves semantically echo each other, with the numerical sequence adding a sense of completeness and logical order, a common pattern in Chinese idiomatic expressions for emphasis and memorability.