Most Open IE systems extract binary relations using domain-independent syntactic and lexical constraints.
systems specialized in other syntactic constructions were also developed, such as noun-mediated relations (Pal and Mausam, 2016), n-ary relations (Akbik and Löser, 2012), nested propositions (Bhutani et al., 2016) and numerical Open IE (Saha et al., 2017a).