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The Tokyo String Quartet and Oscar
The Tokyo String Quartet have appeared on Sesame Street in at least two segments.
One involves the ensemble tuning up outside of Oscar's trash can. Oscar loves the dissonant sounds of the instruments, but when they start playing a classical piece, he's disgusted and asks them to tune up again.
Another is a segment in which the quartet are shot against a plain pink background and an announcer counts them as they disappear and reappear in frame.