When you’re writing a song, you may start by settling on a base chord progression. This is the heart of the song. You may deviate from it a bit along the way, but it always comes back to that progression.
The problem is that there are only so many progressions, and musicians often use the most popular ones over and over again. There is even a famous comedy skit where a man plays Pachelbel’s Canon in D and then sings major pop and rock hits over it. The skit is meant to be funny, but it also shows just how many different songs can fit that same basic progression.
Songs may end up sounding similar
The problem that this creates is that two songs, especially in the same genre, may sound a bit similar. A musician may not intend to copy the first song, and the artist might not have even heard the other song before. It’s just a natural result of the fact that thousands upon thousands of songs come out every year that many of them will use the same dozen chord progressions in at least some fashion.
This can make lawsuits over plagiarism very complicated. It’s unlikely that two people would write the same book, and an exact match of the text would show that it was intentional. But two artists could likely use the same chords, and it not have been intentional at all. This can lead the first artist to claim they were being copied when the second has never heard of them or listened to their song, creating a contentious divide.
If you find yourself in this position with your music, you need to know your legal options.