I propose disabling mash-up mixes in music plagiarism cases.
For now in music plagiarism trials it's a usual and accepted way of proving substantial similarity that the complaining party prepares mash-up recordings. In these mash-ups they combine the backing track and the lead vocals of the compared songs in various ways. If the mash-up "works" it certainly influences jurors to vote for the existence of substantial similarity.
The main point of this article that mash-ups should be disabled to use in courtrooms. Reason: it is able to mislead jurors by creating a false impression of similarity. The mash-up trick also works for songs that are VERY different both melody-wise and harmony-wise, for certain extent. The mash-up mixes often apply an adjustment of pitch and tempo in a smooth way. The difference of key and exact tempo (+/-30%) is not problem for mash-up producers.
For now there exists huge amount of mash-up mixes supporting this point. Just type "mashup" into YT search and check the results! There are some shocking examples for example Smells Like Teen Spirit - Final Countdown. Here we have to add that the mashup would not work with the chords of Teen Spirit combined with the melody of Countdown.
Pretty Woman - Sweet Child Of Mine:
Roy Orbison junior in 2018 shared his opinion that the intro guitar hook of Sweet Child Of Mine can be mashed up with Pretty Woman (written by his father). He added that he does not think it's a case of plagiarism together with other songs that too are allegedly similar to Pretty Woman: Day Tripper, Staisfaction, etc...
Blurred Lines vs. Got To Give It Up:
It is another example for two different songs with different chord progression and modal pitches (b3rd degree vs 3rd degree) and it still can be mashed up for certain degree. Not perfectly since some clashing notes creat painful dissonances at more points.
When do mashups work? It's easier to determinine when it does not work.
A given chord works with a melodic subphrase when the "anchoring" note(s) of this subphrase is present in the chord (there are counter examples). Mash-up mix does not work good, if an anchoring note (usually one note per subphrase, sometimes more) in Song A doesn't fit with the corresponding chord in Song B. A seventh or major seventh dissonance is still creates an "enjoyable" dissonance, but a 2nd melodic degree of an anchoring note in Song A would be clashing with the tonic chord of Song B, since that chord contains 1st and 3rd degrees neighbouring the 2nd degree. This same note (2nd degree) would not clash with either ii, iii(7) or the V chord (the latter is called "dominant" chord) in Song B. Roughly a half of the basic chords don't clash with a given scale degree (1st to 7th).
Phrase lengths also count, but these have usually "binaric" length: 4, 8, 16 or 32 bars.
Another typical clashing is caused by modal differences. For example a modal (including minor mode) flat-3rd degree of Song-A is clashing with the natural 3rd degree built in the actual chord of Song-B. This type of clashing occures in the Blurred Lines - Got To Give It Up mashup.