In metrology and other sciences, such as chemistry and physics, a standard is a reference that is used to calibrate measurements. Historically, each authority defined its own standards for systems of weights and measures. This resulted in confusion. Although some of the older systems are still in use, modern standards are internationally recognized and defined under controlled conditions. In chemistry, for example, a primary standard may be used as a reagent to compare purity and quantity in a titration or other analytical technique. In metrology, a standard is an object or experiment that defines the unit of a physical quantity. Examples of standards include the international prototype kilogram (IPK), which is the mass standard for the International System of Units (SI), and the volt, which is the unit of electrical potential and is defined based on the output of a Josephson junction. There are different levels of standards for physical measurements. The master standards or primary standards are those of highest quality, which define their unit of measure. The next level of standards in the hierarchy are secondary standards, which are calibrated with reference to a primary standard. The third level of the hierarchy encompasses the working standards. Working standards are periodically calibrated from a secondary standard. There are also laboratory standards, which are defined by national organizations to certify and calibrate labs and educational facilities. Because laboratory standards are used as reference and are held to a quality standard, they are sometimes (incorrectly) referred to as secondary standards. However, that term has a specific and different meaning.