First off, technically everything is copywritten as soon as it is published. The Copyright Office and Library of Congress (or the equivalent in other countries) just make it easier to prove who was the originator of the work.
A piece of music, literature, photography... is officially published once one other person besides the originator has read, seen or heard it. So technically, if you post a photo to Facebook and it pops up in any of your friend's news feed, it is published.
Here's where the complicated part comes into play. Weather of not you are in violation of copyright laws or are committing copyright infringement depends on a lot of different factors.
How old is the piece of work? After a certain point, it becomes part of the "public domain" which means it's basically free to use by anyone at any time.
Is the use for "educational purposes?" If it is, then as a general rule it is okay to use a given work so long as you are not costing the originator profits by doing so.
And in some cases, if you contact the originator of the work, he or she might be willing to let you use their piece because it can amount to free promotion for them.
In your particular case, the only way you can be sure that it's OK to use a line from another song in one of your own is if that song is part of the aforementioned public domain or the artist/copyright holder gives permission to do so.
As far as youtube goes, that really blurs the lines and I suggest you check out the following links for more information:
Xenolan answered Wednesday October 16 2013, 2:58 pm: Generally speaking, copyright comes into play when someone records music and then sells the recordings. You can sing covers live without fear of copyright issues, for instance.
A single line isn't going to be an issue, either. Peter Schilling's "Major Tom" borrowed very heavily from David Bowie's songs about an astronaut named Major Tom, and my understanding is that Bowie considered legal action but was advised by his lawyers that he couldn't win. Katy Perry sang "California Girls" and the Beach Boys couldn't sue her either. In order to be a copyright violation, the new song must be so similar to the original that one could actually mistake the new one for the old.
5 seconds of a song on YouTube isn't going to get you sued either. That falls under "Fair Use". It would be like quoting a single page out of a book.
As you say, "how do they truly know it's their song?" The fact is that they don't. So many songs have been recorded over time, I would imagine that it's just about impossible to write a new song where every line is original.
Bottom line is that no musician is going to get uptight over one line from their work being used again, even if it's very memorable. Most would be happy to see that they are inspiring other artists. [ Xenolan's advice column | Ask Xenolan A Question ]
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