*Definition [#j712cd0a]
-A topological space (X,τ) is said to be pre-D_1 if for any distinct pair of points x and y of X, there exists a [[pD-set>pre-Difference set]] of X containing x but not y and a [[pD-set>pre-Difference set]] of X containing y but not x.

*Property [#u2696f9b]
If (X,τ) is pre-D_1 , then it is [[pre-D_0]].

*Reference [#z9b21e6a]
-Caldas, M.; Georgiou, D. N.; Jafari, S. , ''Characterizations of low separation axioms via α-open sets and α-closure operator.'', (English summary) Bol. Soc. Parana. Mat. (3) 21 (2003), no. 1-2, 97–111.