adjective
1
BAD : I prefer classic music to pop.
GOOD : I prefer classical music to pop.
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classic = being among the best or most typical of its class; serving as a standard or model: 'The painting is a classic example of sixteenth-century Venetian art.'
classical music = the music of Mozart, Beethoven, etc
2
BAD : I love the sound of a classic guitar.
GOOD : I love the sound of a classical guitar.
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a classical guitar = a guitar that is used to play classical music