The Illinois Senate is the upper chamber in the Illinois General Assembly, the legislative branch of the government of the state of Illinois. Created in 1818 by the constitution of the first state, the Illinois senate is comprised 59 senators elected from individual legislative districts determined by population. According to Illinois' 1970 constitution, senators are divided into three groups, each group having a two-year term at a different part of the decade between censuses, with the rest of the decade being taken up by two four-year terms. Depending on the election year, roughly one-third, two-thirds, or all Senate seats may be contested. In contrast, the Illinois House of Representatives is made up of 118 members with its entire membership elected to two-year terms. House districts are formed by dividing each Senate district in half.