Unions of left-separated spaces

A space is left-separated if it has a well ordering for which initial segments are closed. We explore when the union of two left-separated spaces must be left-separated. We prove that if X and Y are left-separated and \({X \cup Y}\) is locally countable, then whenever \({{\rm ord}_\ell(Y ) \leq \omega_{1}, X \cup Y}\) is left-separated. In 1986, Fleissner 2] proved that if a space has a point-countable base, then it is left-separated if and only if it is \({\sigma}\)-weakly separated. We provide a new proof of this result using elementary submodels and add an additional characterization.