I Never thought about it before....

[L Verge]

I did a little searching after posting and found that the terms literally applied to where the legislators sat, and the practice continued, based chiefly in France. By 1848, they were termed "democratic socialists" on the right and "reactionaries" on the left.

When the Third Republic came in in 1871, the terms were adopted by political parties -- a bunch of them: Republican Left, Centre Left, Centre Right, Extreme Left (1876) and Radical Left (1881) -- again, this is in France. Beginning in the early twentieth century the terms left and right came to be associated with specific political ideologies and were used to describe citizens' political beliefs, gradually replacing the terms "reds" and "the reaction" or "republicans" and "conservatives". By 1914 the left half of the legislature was composed of Unified Socialists, Republican Socialists and Socialist Radicals, while the parties that were called "left" now sat on the right side

In Britain, the terms "right-wing" and "left-wing" came into politics in the late-1930s when the government was debating the Spanish Civil War. The U.S. appears to have been a late-comer in the political name-calling arena.