De Wahl 's rule is a rule of word formation , developed by the linguist Edgar de Wahl and applied in a constructed language Occidental ( known also as `` Interlingue '' ) created by him . The de Wahl 's Rule is a rule of word formation , developed by the Balto-German naval officer and teacher Edgar de Wahl and applied in the artificial language Interlingue ( also called `` Occidental '' ) which was also his creation .