The copyright math is complicated, and even experts disagree commonly. Wikipedia's lists, though helpful, have been known to be disagreed with by the courts on numerous occasions.

For works published before Jan 1, 1978, it generally comes down to:

+ Lifetime of last involved author, +70, or +95 or +120 years.

+ If the work was already under copyright at that date (not everything was), then it also gets extended by 45 years

75 years came from the example given by the official documentation that attempts to explain the various acts (not definitive, but helpful) [0], and how they interact with each other:

> Example: A work that first secured federal copyright pro-tection on October 5, 1907, and was renewed in 1935, would have fallen into the public domain after October 5, 1963. The first act extended the copyright to December 31, 1965; the second act extended it to December 31, 1967; the third act extended it to December 31, 1968; the fourth act extended it to December 31, 1969; the fifth act extended it to December 31, 1970; the sixth act extended it to December 31, 1971; the seventh act extended it to December 31, 1972; the eighth act extended it to December 31, 1974; the ninth extended it to December 31, 1976; and the 1976 Copyright Act extended the copyright through the end of 1982 (75 years from the end of the year in which the copyright was originally secured)

This describes an example published in 1907, and we're talking about one published in 1909. As a rough guide, a minimum of 75 years fits works of that era. It is often actually more.

But, as I didn't put a lot of effort into my earlier statement, let's do a little more digging.

A provision in the UK's 1988 act probably means that Potter's works would have been revived for another fifty years, that is, expiring in 2038. However, that was only if Potter's estate owned the copyright - which it turns out, is not the case.

Potter left her works (and their copyright) to the National Trust upon her death, in the UK, which has a different set of copyright laws that are almost, but not quite fitting to the ones I've described above. (Which is common. The laws look the same until you try and do anything.)

The National Trust _sold_ the copyright (not just the publication right) to a company that was eventually absorbed into the Penguin Group, and so the copyright belongs to them.

This makes it extremely complicated. At one point, the National Trust held the copyright, meaning that Crown Copyright laws applied, which are generally shorter. However, the copyright was then passed on to a private company.

It is difficult to track down a date when the rights were passed from the Trust to Penguin - it may have been that the works had already expired at that point, meaning they might _mostly_ (see 'Another note' below) in the public domain, before considering the new ownership.

If however the work wasn't yet in the Public Domain, then a tweak of the UK copyright laws in 2006 may mean that they still aren't. The "artistic resale" right (literary works fall under artistic copyright in the UK), which is non-reassignable, and non-waivable, says that as long as the work is still being sold in a reasonable number, it both qualifies for royalties (in this case to Penguin), and cannot enter the public domain. However, these sales must come in units of greater than 1,000 to qualify. As a very considerable number of these books are still being sold, it appears to fall under this right.

Another note: Penguin published a previously unpublished work in 2016, containing some of her illustrations. Which, unfortunately, means that Penguin has a definite right to those works and any works that may appear to be derivatives of it. Whilst this new work might be considered separate from previous works, courts have sometimes in the past considered book series, even without overarching plotlines, to be a single work, even across extremely varied timelines. Penguin could argue all works should be linked to the copyright of the new work. (This is an odd situation. If Penguin didn't have the copyright right, and just the publication, then the work would expire in 2039, under the guidance on posthumous works given in the 1988 act. But Penguin has the actual copyright, not just publication rights.) (How does a company having the copyright work out with the general guidance of author + 70 years? It gets simplified. Publication date + 50 years is used instead.)

However, despite all this, for now, Penguin have said they won't enforce their copyright on the works. Copyright in the UK is not (today) a right that can be waived, so it doesn't automatically mean Public Domain. However it does probably mean that you don't have to have a lawyer looking over the situation.

[0] https://www.copyright.gov/circs/circ15a.pdf