At the turn of the 20th century, humidity threatened the reputation of Brooklyn’s Sackett-Wilhelms Lithographic and Publishing Company’s high-quality color printing. After two summers of extreme heat disrupted business and caused swelling pages and blurry prints, the printing company found that a nascent cooling industry could offer help.
Willis Carrier, a 25-year-old experimental engineer, created a primitive cooling system to reduce humidity around the printer. He used an industrial fan to blow air over steam coils filled with cold water; the excess humidity would then condense on the coils and produce cooled air.
It used to mean the end of the 19th, start of the 20th, but it evolved. No need to be snarky, I’m not fighting museums here.
Source: https://en.m.wikipedia.org/wiki/Turn_of_the_century which has additional sources. Unfortunately most sources aren’t clear either (both Cambridge and Webster dictionaries state that it’s when a century ends and another begins, without more info if the century is specified).
I’m just trying to help disambiguate.
It comes off as needlessly pedantic