A musical instrument is an instrument to create musical sounds. Or, an instrument which is musical. But... music instrument is an instrument to create music. Or, an instrument which is used in composing and/or performing music. Which one is correct? Perhaps both. But musical instrument is more common in literature . First-come, first-served game. Instrument is a noun. Musical is the adjective, to describe the noun. Adjective-noun phrase. Like angry monkey , it's a monkey which is angry. While music is also a noun, noun-noun, like anger monkey , which means a monkey appears in anger, that's uncommon. But... EEEYYYYYY.... Therefore, logic dictates we go with: musical instrument Aw... But hear me out. Music instrument is technically legit! It's a noun-noun construction, and in theory, it makes sense; an instrument for music. But indeed, musical instrument won the linguistic first-come, first-served game and became the dominant term in English...