De Wahl's rule

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. It is useful to create certain grammatical forms from others (for example substantives from verbs).
Rule.
Infinitives of verbs in the language Occidental have usually the ending -ar or -er. The root is obtained in the following manner: 
These six exceptions are: 
and the verbs formed from them with prefixes.
Because the rule composes from three parts, it is often known as "three de Wahl's rules".
One creates the substantives and adjectives by removing the endings and thus obtaining the root. After adding -r or -er, one obtains an infinitive in a majority of cases: "decora/t/ion, decora/t-, decora/r".
Application.
This rule was applied in the language Occidental. After a possible modification one can apply this rule to create new forms of a word especially in Romance languages or in languages which borrowed vocabulary from Romance languages.