Keira Knightley and Jimmy Fallon Square Off in Musical Showdown (Video)

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Keira Knightley and Jimmy Fallon went head to head and note for note last night on Late Night.

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The host and his Anna Karenina-star guest squared off in a guessing game that tested their musical prowess, as they had to play the songs on random instruments like the accordion or a melodica -- a strange combination of harmonica and piano.

Luckily, they had a little help from musical sensations Dave Matthews and Michael Buble, although their ears didn't turn out to be of much assistance.

“I feel like this exposes me in a professional way that’s unfair,” Matthews cracked.

“Am I the only one that doesn’t actually play an instrument?” Buble asked.

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Knightley went first and had little success with the violin playing “Row, Row, Row Your Boat.” Matthews, her partner, could only guess “Jingle Bells.”

Buble guessed Fallon’s attempt at “Old McDonald Had a Farm” on the melodica in eight seconds. Knightley topped that guess when Matthews played, ironically enough, “Jingle Bells” on the cello. Knightley only took four seconds to guess.

Watch on for the rest of the confrontation.