It is not explained and the footnote is not helpful. Here is the European and Russian explanation as 1789-1914: https://en.m.wikipedia.org/wiki/Long_nineteent

It is not explained and the footnote is not helpful. Here is the European and Russian explanation as 1789-1914: https://en.m.wikipedia.org/wiki/Long_nineteenth_century https://en.m.wikipedia.org/wiki/Long_nineteenth_century In Britain, it is not long, but slightly shifted, to mean 1815-1914, roughly the Pax Britannica: https://en.m.wikipedia.org/wiki/Pax_Britannica https://en.m.wikipedia.org/wiki/Pax_Britannica In similar fashion, the 20th century is often represented as short, meaning from the Russian Revolution or WWI to the fall of the Soviet Union 1914/18-1989/91: https://m.wikidata.org/wiki/Q3769095 https://m.wikidata.org/wiki/Q3769095