Speaking as an attorney who has been called on to do copyright research now and again–but who is not a copyright attorney as such–I would say that Cliffy 's explanation is pretty well on track.
There are films so old that their initial copyright term expired, and their renewal term expired too. In America, these are from 1924 and before.
There are films for which initial term of copyright expired, and for which a timely renewal was not made. This was the case with Till the Clouds Roll By, a film about the life of Jerome Kern, starring Robert Walker.
There are films for which the copyright holder was a legal entity which went belly-up without transferring the rights. This was the case with many films from the British company Gaumont, such as the Alfred Hitchcock version of The 39 Steps.
In 1975 a copyright reform act extended the copyright duration of films then still under copyright. This was the act which initiated the statutory life but 50 years provision. Sonny Bono sponsored a bill which changed the 50 years to 70.
The lastest I have read on the subject–a textbook on copyright published about three years back–the copyright status of It’s a Wonderful Life is uncertain; basically, people have been scared out of running it on account of threats of litigation by Spelling Entertainment.
There is a further complication. (Actually, there are probably a lot of further complications). If a copyright holder contracted with a distributor so as to give them some exclusive rights to the movie while it was still under copyright, and the copyright has since expired, but the licensing contract has not, and the film was based on some pre-existing work which was under copyright at the time the film was made, the film will be treated as being under copyright, and the exclusive contract will still be considered valid, until the copyright on the original, under-lying work expires.
I expect this may be as clear as mud. Perhaps a brief discussion of the case where this rule was established will make things clearer:
Gaumont Studios made a film of Pygmalion with Leslie Howard and Dame Wendy Hiller. Before Gaumont went out of business, it entered into a contract with Janus Films giving it exclusive rights to distribute the movie. When Pygmalion went into the public domain, a company called Budget Films began advertising it in its film catalog. Janus sued, pointing out that the only reason Gaumont had been able to have a copyright in the movie was that it got permission from the owner of the copyright on George Bernard Shaw’s play called Pygmalion in order to shoot the movie. This play was still under copyright when Budget Films put the movie in its catalog, even though the movie was not. It was ruled that Janus still had exclusive rights to the distribution of the movie, exactly as though they were the holder of an unexpired copyright, until the original play’s copyright expired.
A company called Nolo Press has published several very good texts on explaining the practical ins and outs of copyright, and, in particular, on using material which is in the public domain.