Why do we use the terms right and left in politics to identify ideologies or concepts on a one-dimensional axis? It should be borne in mind that not in all countries do both terms have the same meaning. Generally in Europe the right is associated with conservatism and the left with socialism and communism, but for example in the United States the Democratic Party is often referred to as the left.
Everything has its origin in the French Revolution and, more specifically, in the convocation of the States General by King Louis XVI on May 5, 1789. The States General were an assembly, created by Philip IV in 1302, which brought together representatives from each of the estates of the Old Regime: the clergy (first state), the nobility (second state) and the representatives of the cities that had a consistory (third state). In the 487 years that passed between their first convocation and the last one in 1789 they only met 21 times, always because of some financial or political crisis.
Each of the three states had approximately the same number of representatives, but the vote was collective, which in practice meant that the nobility and clergy, traditionally allies, always imposed their views. But on December 27, 1788, the French Council of State had taken the unprecedented decision to double the number of representatives from the third state.
Aware of their majority, the members of the third state, formed by the bourgeoisie and the common people, asked at the May 1789 meeting for each representative to have one vote. They had 578 delegates compared to 291 from the clergy and 270 from the nobility, i.e., an absolute majority. However, the request was denied (two state votes against one) with the approval of the king.
At that meeting, and from the point of view of the President of the Assembly, the nobles sat on the right and the bourgeois sat on the left. The former defended the interests of the aristocracy and the monarchy, while the latter supported republicanism and civil liberties. However, since the third state was composed mainly of bourgeois and merchants, they also defended free trade and capitalist policies, something associated today with the right wing.
In any case, the conceptual division prospered and when on August 28 that same year the representatives of the third state and the low clergy attending the States General declared themselves redefined as the National Assembly (initiating the French Revolution), promising not to separate until a constitution had been drafted, they were equally distributed to the right and left of the presidency.
This occurred on the occasion of the vote on an article of the new constitution, which established the king’s power to ban any law passed by the future Legislative Assembly. Those in favour stood to the right of the president, while those against stood to the left.
Moreover, when the Legislative Assembly met for the first time on 1 October 1791, the members of the Club des Feuillants (who were in favour of a constitutional monarchy) and the Girondins (representatives of the great Republican bourgeoisie) sat on the right, while the Jacobins (representatives of the petty bourgeoisie) and the members of the Club des Cordeliers (representatives of the common people) sat on the left.
According to Christopher Cochrane, it was completely accidental that the royalists sat on the right and the revolutionaries on the left. If they had sat upside down, today the right would be the left and vice versa. So the terms left and right are completely arbitrary. What was not arbitrary was that the people sitting on each side chose to sit with certain people, and against others. A member of the States General in 1789 wrote: I tried to sit in different places each time, but I was forced to stop sitting on the left if I did not want to vote alone or be booed from the gallery.
Over time, the definition of the political right and left underwent changes and transformations, mainly due to the new political movements that emerged especially from the 19th century. Sometimes it is difficult to establish an exact definition of both, since what in a historical moment is classified in one term may be the opposite in another.
Thus, with the development of the capitalist economies, the bourgeoisie representing the original left ended up replacing the aristocracy and occupying the right, while the working class, increasingly numerous and organized through trade union, socialist and communist policies, filled the gap on the left.
Sources: The Government and Politics of France (Andrew Knapp, Vincent Wright) / Left and Right: The Small World of Political Ideas (Christopher Cochrane) / Encyclopaedia Britannica / Wikipedia.