The word "教科书" is formed by sequentially combining three characters: 教 (jiào), meaning "to teach", 科 (kē), meaning "subject" or "course", and 书 (shū), meaning "book". These characters are joined in a logical order where "teach" modifies "subject", and together they modify "book", resulting in a compound term that directly denotes a book used for teaching a specific subject, i.e., a textbook.