units that are accepted for use with the SI". See Non-SI units mentioned in the SI for a full list. Most of these, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min ), the hour (), and the day (); the degree (for measuring plane angles, and the electronvolt (a unit of energy, New units The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. Defining magnitudes of units Since 2019, the magnitudes of all SI units have been defined in an abstract way, which is conceptually separated from any practical realisation of them. Namely, the SI units are defined by declaring that seven defining constants have certain exact numerical values when expressed in terms of their SI units. Probably the most widely known of these constants is the speed of light in vacuum, , which in the SI by definition has the exact value of = . The other six constants are , the hyperfine transition frequency of caesium; , the Planck constant; , the elementary charge; , the Boltzmann constant; A, the Avogadro constant; and cd, the luminous efficacy of monochromatic radiation of frequency . The nature of the defining constants ranges from fundamental constants of nature such as to the purely technical constant cd. Prior to 2019, , , , and were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. As far as realisations, what are believed to be the current best practical realisations of units are described in the , which are also published by the BIPM. The abstract nature of the definitions of units is what makes it possible to improve and change the mises en pratique as science and technology develop without having to change the actual definitions themselves. In a sense, this way of defining the SI units is no more abstract than the way derived units are traditionally defined in terms of the base units. Consider a particular