This was achieved by dividing the senators of the 1st Congress into thirds (called classes), where the terms of one-third expired after two years, the terms of another third expired after four, and the terms of the last third expired after six years. This arrangement was also followed after the admission of new states into the union. The staggering of terms has been arranged such that both seats from a given state are not contested in the same general election, except when a vacancy is being filled.