The word 应该 is formed through the combination of two individual characters, each contributing its core conceptual meaning. The first character, 应 (yīng), derived from a traditional character meaning "to answer" or "to correspond", carries an inherent sense of fitting a situation or giving a proper response. The second character, 该 (gāi), historically relates to notions of necessity, obligation, or what is incumbent. Over time, these two concepts were fused into a single compound word where the idea of a "proper correspondence" (应) merges with the idea of "necessity" (该) to create the modern meaning of "should" or "ought to".