The left-right classification comes from the National Assembly
just before the French Revolution
. Members who supported political rights for all classes of society, would sit on the left side of the assembly hall in which they met. Members who supported the monarchy, would sit towards the right. Those who had been offended by their own party would frequently stand up and walk over to the other side of the hall in order to make a political statement. Historian Arthur M. Schlesinger, Jr.
has said the spectrum theory was "adequate to the political simplicities of the nineteenth century, when the Right meant those who wished to preserve the existing order and the Left meant those who wished to change it. But the twentieth century, here as elsewhere, introduced new ambiguities." 
- ↑ Not Right, Not Left, But a Vital Center, Arthur M. Schlesinger, Jr., New York Times Magazine, April 4, 1948.