State Question 640 was a citizen-initiated ballot measure that was approved by Oklahoma voters in a special election in March 1992 with 56.2 percent of the vote. The measure amended Article 5, Section 33 of the Oklahoma Constitution to add restrictions on how revenue bills can become law. Under SQ 640, a revenue bill can only become law if: (1) it is approved by a 3/4th vote of both legislative chambers and is signed by the Governor; or (2) it is referred by the legislature to a vote of the people at the next general election and receives majority approval. State Question 640 also prohibited a revenue bill from containing an emergency clause; instead, revenue bills can only take effect 90 days after being signed by the Governor.
Since passage of SQ 640 in 1992, Oklahoma voters have approved only one state question to raise taxes: SQ 713, which increased the tobacco tax in 2004. Until passage of HB 1010xx in 2018, no revenue bill succeeded in gaining approval from three-quarters of legislators in both chambers.