Mass. Gen. Laws ch. 107, § 3

If there is no agreement or provision of law for a different rate, the interest of money shall be at the rate of six dollars on each hundred for a year, but, except as provided in sections seventy-eight, ninety, ninety-two, ninety-six and one hundred of chapter one hundred and forty, it shall be lawful to pay, reserve or contract for any rate of interest or discount. No greater rate than that before mentioned shall be recovered in a suit unless the agreement to pay it is in writing.