The New Jersey Constitution of 1947 (Annotated) Art. XI, § II, ¶ 3

3. Each member of the Senate elected in the years one thousand nine hundred and forty-five and one thousand nine hundred and forty-six shall hold office for the term for which he was elected.  Each member of the Senate elected in the year one thousand nine hundred and forty-seven shall hold office for a term of four years beginning at noon of the second Tuesday in January following his election.  The seats in the Senate which would have been filled in the years hereinafter designated had this Constitution not been adopted shall be filled by election as follows:  of those seats which would have been filled by election in the year one thousand nine hundred and forty-eight, three seats, as chosen by the Senate in the year one thousand nine hundred and forty-eight, shall be filled by election in that year for terms of five years, and three, as so chosen, shall be filled by election in that year for terms of three years, and those seats which would have been filled by election in the year one thousand nine hundred and forty-nine shall be filled by election in that year for terms of four years, so that eleven seats in the Senate shall be filled by election in the year one thousand nine hundred and fifty-one and every fourth year thereafter for terms of four years, and the members of the Senate so elected and their successors shall constitute one class to be elected as prescribed in paragraph 2 of Section II of Article IV of this Constitution, and ten seats shall be filled by election in the year one thousand nine hundred and fifty-three and every fourth year thereafter for terms of four years, and the members of the Senate so elected and their successors shall constitute the other class to be elected as prescribed in said paragraph of this Constitution.