Audio and moving images are set apart from any other content by the adherence to a time line. As with any other media, we may state globally what a story is about. If we want to be more specific, we can state this by chapter or section of a text for anything that is written on paper. To make an equivalent statement about the songs of an Album, or if we want to locate King Marke finding Tristan and Isolde red-handed, which happens from bar 101 in the second act of Wagner's iconic opera, we need to align this i.e. with time elapsed since the start.