*Definition [#na5033b1]
A closed subbase S for a T_1 space X is said to be binary if for all &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b$M%5csubset%20S$%7d%25.png); with &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b$%5ccap%20M=%5cemptyset$%7d%25.png); there exist A and B in M with &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b$A%5ccap%20B=%5cemptyset$%7d%25.png);.
*Reference [#l7fb20c8]
J. van Mill and E. Wattel, ''An external characterization of spaces which admit binary
normal subbase'', Amer. J. Math. 100(1978), pp987-994.