Calibration and test weights are subject to standards of mass certified by international or national laboratories like NIST, ANSI/ASTM, or ISO. These weights are typically based on the international kilogram, but other options do exist. Their precise nature makes them suitable for calibrating scales, off of which other objects will be weighed. They are typically called class F weights, which have restrictions ontolerance, material, surface finishes, design, cavities, and adjustments. The weights may have certificates to satisfy quality standards.