Bitopological space

In mathematics, a bitopological space is a set endowed with two topologies. Typically, if the set is ${\displaystyle X}$ and the topologies are ${\displaystyle \sigma }$ and ${\displaystyle \tau }$ then the bitopological space is referred to as ${\displaystyle (X,\sigma ,\tau )}$. The notion was introduced by J. C. Kelly in the study of quasimetrics, i.e. distance functions that are not required to be symmetric.