Missouri constitution
Article III: legislative department
Section 11

The first election
of senators and representatives under this constitution, shall be held at the general
election in the year one thousand nine hundred and forty-six when the whole number
of representatives and the senators from the districts having even numbers, who shall
compose the first class, shall be elected, and two years thereafter the whole number of
representatives and the senators from districts having odd numbers, who shall compose
the second class, shall be elected, and so on at each succeeding general election.