Article 15 opens Chapter 6 of the Decree-Law on the calculation of Due Tax. It provides that the Payable Tax owed by a Taxable Person for any Tax Period is calculated as the Due Tax from that Taxable Person for the Tax Period, less the total Deductible Tax calculated under the provisions of Article 16 of the Decree-Law. The Article therefore establishes a net basis of accounting for each Tax Period: the excise liability arising in the period is reduced by the deductions the following Article allows, and only the resulting balance constitutes the Payable Tax owed to the Authority.