The article 10.1 of the LIS establishes that the taxable base will be constituted by the amount of the income obtained in the tax period minus the compensation of negative tax bases from previous tax periods.
Likewise, Article 25 of the LIS regulates the capitalisation reserve whereby some taxpayers may apply a reduction in the pre-tax base of 10 per cent of the amount of the increase in their own funds.
Therefore, when negative tax bases from previous tax periods are offset (box ), and/or the capitalisation reserve (box ) is applied to the previous tax base (box ), the amount to be entered in box  "Tax base" on page 13 of form 200, will be the result of carrying out the following operation:
 =  -  - 
A tener en cuenta:
If the result of this operation is zero, the figure zero ("0") shall be entered in box  and the other boxes following it shall be treated in the same way as indicated in the following paragraph for the case of a zero amount in box .
If the amount in box  is zero or negative, zero ("0") or the amount preceded by a minus sign (-) must be entered in both box  and box .Likewise, zero ("0") must be entered in boxes  "Total tax liability",  "Positive adjusted total tax liability" and  "Positive net tax liability" on page 14 of form 200, going directly to fill in, where appropriate, the boxes corresponding to withholdings (boxes  to ) on page 14a of the said form.
If the tax base for the period is negative (box ), the amount may be offset against positive income in subsequent tax periods.