31 CFR 601.2 - Description of paper.
The paper utilized in the printing of United States currency and public debt issues is cream-white bank note paper which must contain security features prescribed by the Secretary of the Treasury. All currency paper shall contain distinctive fibers, colored red and blue, incorporated in the body of the paper while in the process of manufacture and evenly distributed throughout. In addition to distinctive red and blue fibers, currency paper shall contain, for denominations prescribed by the Secretary of the Treasury, security threads embedded beneath the surface of the paper during the manufacturing process. Security threads shall contain graphics consisting of the designation “USA” and the denomination of the currency, expressed in alphabetic or numeric characters. In addition to the security thread, for the denominations prescribed by the Secretary of the Treasury, the paper will bear a watermark identical to the portrait to be printed on the paper.