Delannoy is a relatively common name from northern France, Lannoy is a commune, or more commonly a place (modern french l'aulnaie) planted with aulnes (Alders).

Delannoy is a relatively common name from northern France, Lannoy is a commune, or more commonly a place (modern french l'aulnaie) planted with aulnes (Alders). Same meaning, different graphy, for Delaunay, as in Delaunay triangulation.