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There is a long-standing myth in the music industry that the old-fashioned fix of recording a song, placing it in an envelope and mailing it to oneself guaranteed copyright. The post date on the stamp was supposed to serve as proof of the date of origin of the song, provided the envelope remained sealed. However, this method didn't stand up in various court cases and has since been discredited. Moreover, given that your copyright exists on creation, and that an envelope's seal can be carefully unsealed and resealed, this method does seem rather fallible. If your country is a member of the Berne Convention, copyright in a song comes into existence when you create it. It does become more complicated when there are several creators who have contributed to the song, but there are rules governing these "layers." It's best to seek legal advice in that situation. The U.S. copyright office is the only one among the Berne Convention's member countries offering a method to register content of the song (lyrics, melody, chord, etc.). Unfortunately in all other countries, only the title of the song is recorded. The value of the protection afforded is significantly reduced. However, your possession of a dated original is all the proof of authorship that you need in most countries, assuming there is any dispute.
Avoid poor man's copyright. Be aware of the Berne Convention.