Short version, follow the chart here:
http://www.sunsteinlaw.com/practices/copyright-portfolio-development/flowchart.htmBasically, copyright used to give you twenty-eight years of protection. During the expiration year, you could renew the copyright for another twenty-eight years, for a total of fifty-six years.
In 1978, the Copyright Act of 1976 went into effect and kicked this nice little system in the groin, extending copyrights, adding "Life+X" rules, and so forth. There was also a later extension (the Sonny Bono Copyright Term Extension Act) that kicks harder, but isn't relevant to anything here except the continued absence of DC and Marvel books.
Regardless, if you do the subtraction, you see that 1978 minus 56 says that everything published in or before 1922 has absolutely expired (with some weird exceptions--technically, 1909 is the only absolute date that doesn't require additional information), which is where your number comes from. However, some owners may have missed their renewal, so copyrights from as late as 1963 (don't ask) may have also expired. (There's also an extensive field, here, regarding the "standing" to renew--some renewals are made by people who don't have a legal claim to the original copyright, making those renewals suspect, but that's another story.)
Additionally, a valid copyright notice (in the form "Copyright 19xx Owner," where the word could be replaced by the circled-c or "Copr.") was required for a copyright as late as 1989, extending the public domain that way, as well.
(Well...Lastly, there are also cases where the copyright is valid, but the owner is gone with no heirs; technically, it would be infringement to carry such things, but nobody would have standing to sue for damages. But that's a far trickier situation, and not something really worth talking about, if one isn't up to hiring investigators.)