以内
いない · inai · JLPT N4 · noun
within
📏 以内 = from (以) + inside (内): from the limit inward
以 marks a reference point and 内 means inside, so 以内 is everything on the inner side of a limit. Pairs with 以外 (outside of), 以上 (and above), 以下 (and below). Used after a number: 三日以内 = within three days.
以 — phonetic 㠯 (plow/tool) + 人 (person)
Originally a pictograph of a hand-held plow or tool (㠯), later combined with 人 (person) to suggest 'using' something. The core meaning 'to use' extended metaphorically to 'by means of', 'with', and 'because of'. In modern Chinese it functions mainly as a grammatical particle.
内 — enclosure 冂 + enter 入
内 originally depicted something entering (入) into an enclosed space (冂), giving the core sense of 'inside' or 'within'. The pictographic image of going into a covered area extended naturally to meanings like inner, internal, and inland. It contrasts with 外 ('outside').