I assume you're referring to the rule that works from before 1923 are all in the public domain. Apparently the rule is all works that were copyrighted before 1923 are in the public domain.
A work is considered to have been copyrighted before 1923 if it was published:
* in an authorized publication
* in the United States
* before 1923
* and had a proper copyright notice
The sticking point here is that the purported owners of the copyright claim that the 1911 publication of Happy Birthday to You was unauthorized. Therefore, since the first authorized publication was in 1935, the copyright should run from that date.
A work is considered to have been copyrighted before 1923 if it was published:
* in an authorized publication
* in the United States
* before 1923
* and had a proper copyright notice
The sticking point here is that the purported owners of the copyright claim that the 1911 publication of Happy Birthday to You was unauthorized. Therefore, since the first authorized publication was in 1935, the copyright should run from that date.
http://blog.librarylaw.com/librarylaw/2009/07/the-myth-of-th...