The de Wahl's Rule is a rule of word formation, developed by the Balto-German naval officer and teacher Edgar de Wahl

Edgar de Wahl

Edgar von Wahl or Edgar de Wahl was a teacher and creator of the language Occidental...

 and applied in the artificial language Interlingue (also called "Occidental") which was also his creation.

The rule served for the formation of certain changed grammatical forms, like adjectives and nouns, from verb infinitive.

The Rule

Verb infinitive

Infinitive

In grammar, infinitive is the name for certain verb forms that exist in many languages. In the usual description of English, the infinitive of a verb is its basic form with or without the particle to: therefore, do and to do, be and to be, and so on are infinitives...

s in Interlingue most frequently end in -ar or -er. The root is obtained by the following way:

1. If, after the removal of -r or -er of the infinitive, the root ends in vocal, the final -t is added or the final y is changed into t: crea/r, crea/t-, crea/t/or; atiny/er, atin/t, atin/t/ion
2. If the root ends in consonants d or r, they are changed into s: decid/er, deci/s-, deci/s/ion
3. In all other cases, with six exceptions, the removal of the ending gives the exact root: duct/er, duct-, duct/ion.

These six exceptions are

1. ced/er, cess-
2. sed/er, sess-
3. mov/er, mot-
4. ten/er, tent-
5. vert/er, vers-
6. veni/r, vent-

and the verbs formed out of them using prefixes.

Because the rule is actually made of three parts, it also known as the "three rules of de Wahl".

The nouns and adjectives are created by removing the ending and thus obtaining the root. After adding -r or -er, one obtains the infinitive in the majority of cases: decora/t/ion, decora/t-, decora/r.

Application

This rule is 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.