The word "一朝一夕" is formed by sequentially pairing the numeral "一" (one) with "朝" (morning) and then again "一" with "夕" (evening), creating a symmetrical four-character structure where each "一" directly modifies its respective time-related character; this literal juxtaposition of "one morning" and "one evening" combines to express the idea of a short duration, specifically a single day or a brief period.