De Wahl's rule is a rule of word formation, developed by Baltic German naval officer and teacher Edgar de Wahl and applied in the constructed language Interlingue, which was also his creation.
The rule served for the formation of certain changed grammatical forms, like adjectives and nouns, from verb infinitive.
Edgar de Wahl observed existing patterns of sound changes that occurred in natural languages (d to s, r to t, etc.). The purpose of his rule was to distill these patterns into a regular and logical system that is reproducible yet also natural in appearance.