Where a serious search for the rightsholder is unsuccessful, their work may not be used from a strictly legal point of view. Fortunately, the copyright exceptions sometimes apply to certain uses in education. If no exception applies, works for which the rightsholders can't be found (orphan works) are often used in practice. In that case, a disclaimer is added to the product or website that includes the orphan work. It states that the institution has made every effort to locate the rightsholders of the works used, but has not been successful (in all cases). Then the address the rightful rightsholders...
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