Mass. Gen. Laws ch. 59 § 5C 1/2
In a city or town that accepts this section and is certified by the commissioner to be assessing all property at full and fair cash valuation, a taxpayer who otherwise qualifies for an exemption pursuant to any clause specifically listed in the first paragraph of section 5 for which receipt of another exemption on the same property is prohibited, shall be granted an additional exemption that shall be uniform for all exemptions and the amount of which shall not exceed 100 per cent of the exemption for which the taxpayer qualifies, as may be determined by the legislative body of the city or town, subject to its charter, not later than the beginning of the fiscal year to which the tax relates. Notwithstanding any provision of this chapter to the contrary, the exemption shall be in addition to any exemption allowable pursuant to said section 5; provided, however, that in no instance shall the taxable valuation of the property, after all applicable exemptions, be reduced below 10 per cent of its full and fair cash valuation, except through the applicability of clause Eighteenth of said section 5; and provided, further, that the additional exemption shall not result in any taxpayer paying less than the taxes paid in the preceding fiscal year. Acceptance of this section by a city or town shall not increase the amount that it otherwise would have been reimbursed by the commonwealth pursuant to the respective clause.
Mass. Gen. Laws ch. 59, § 5C 1/2