The custom of dividing the Senate desks by political party is almost as old as the parties themselves. Democrats traditionally sit to the presiding officer's right; Republicans to the left. But the division has not always been as definitive as it is today. In the Old Senate Chamber (1810-1859), an equal number of desks were placed on each side of the aisle, without regard to party size. When one party elected more than half the senators, some majority party members had to find space on the minority party side. When the Senate moved to the current Chamber in 1859, the practice of dividing the desks equally continued for several years even though the new Chamber was large enough to permit a flexible seating arrangement. Finally in 1877 the practice began of moving desks back and forth across the center aisle to permit all majority party members to sit together on the appropriate side, except in a few cases when an unusually large majority existed.
Senators independent of either party have traditionally chosen for themselves which side of the aisle to sit. Once, during the 1950s, when Senator Wayne Morse of Oregon had left the Republican Party but not yet joined the Democrats, he placed his chair temporarily in the middle of the center aisle to demonstrate his independence.