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Whistling was Toots Thielemans' signature non-instrumental musical technique. He was a renowned jazz harmonica player, but his whistling skills elevated his performances, adding a unique and expressive layer to his music. Thielemans' whistling was characterized by its melodic intricacy, rhythmic precision, and ability to convey emotion. It became an integral part of his jazz style, earning him the nickname "The Whistler."