The Planck units are physical units of measurement. The system of these units was first developed by Max Planck. The definition of the five base Planck units are based only on five physical constants found in nature. When Planck units are used to express any of these five physical constants, the value is 1. This allows physicists to simplify many equations about physical law. Planck proposed these units in 1899. They are also known as "natural units" because the origin of their definition comes only from properties of nature and not from any human construct. Planck units are only one system of natural units among other systems. They are considered unique, because these units are not based on properties of any prototype object, or particle (that would be arbitrarily chosen) but are based only on properties of free space. The constants that Planck units, by definition, normalize to 1 are the: