Nonstandard analysis deals primarily with the [[hyperreal number|hyperreal]] line. The latterhyperreal line is an extension of the real line, whichcontaining containsinfinitesimals, in addition to the reals, also infinitely small quantities. In the hyperreal line every real number has a collection of numbers (called a [[monad (mathematics)|monad]]) of hyperreals infinitely close to it. The standard part function associates to a [[Wikt:finite|finite]] [[hyperreal number|hyperreal]] ''x'', the unique standard real number ''x<sub>0</sub>'' which is infinitely close to it. The relationship is expressed symbolically by writing
