Article 156 imposes an 'Additional Tax' (late payment penalty) of 1% per month on any outstanding tax that remains unpaid after its due date. This penalty is calculated monthly for as long as the tax remains unpaid. This acts as a deterrent against delays and compensates the state for the lost time-value of the revenue. However, the Law provides the Chairman with the authority to grant relief—either partial or total—from this additional tax, according to the specific rules and circumstances defined in the Executive Regulations of the Law.