What does the phrase "the turn of the nineteenth century" mean?

Solution 1:

The fact that the Chicago style manual has no clear definition reflects, I think, the fact that this is a fundamentally confusing term, due to a confluence of two sources of confusion:

"turn of the century" - does this happen at the end or the start?

"19th century" - is this 1800 to 1899, or 1900 to 1999?

Now, you and I could agree a clear answer to either of these questions (the second is less open to debate), but regardless, many other people will think the other way. It doesn't really matter whether we're right or wrong, if the aim is to create a clear communication, which after all is the primary function of language, whether spoken or written. Context will always help - if you're talking about something that happened in 1803 (eg the start of the Napoleonic Wars in Europe), and you say "diplomatic relations at the turn of the 19th century were at a low point" then people would guess what you mean.

So, I think that, if pushed to provide an answer, the Chicago Style manual would probably side with Wikipedia - that "The turn of the 19th century" means, say, 1795 to 1805. But it will still be a confusing choice of words to many.