In 1992, the voters of the state approved a measure which amended Article X of the Colorado Constitution that restricts revenues for all levels of government (state, local, and schools). Under TABOR, state and local governments cannot raise tax rates without voter approval and cannot spend revenues collected under existing tax rates without voter approval if revenues grow faster than the rate of inflation and population growth. Revenue in excess of the TABOR limit, commonly referred to as the "TABOR surplus", must be refunded to taxpayers, unless voters approve a revenue change as an offset in a referendum. Under TABOR, the state has returned more than $2 billion to taxpayers.