These 'working' languages can be further subdivided into 'active' and 'passive' languages. Below a fuller description is provided of the different terms, following the AIIC classification of A, B and C languages. AIIC members are expected to have at least one A language.
Some interpreters work into B languages in only one of the two modes of interpretation.
In principle, an interpreter's main active language is the mother tongue - the language in which the interpreter was formally educated and feels completely at ease. An active language which is not the interpreter's mother tongue can only be acquired after years of hard work and frequent stays in a country of that language. Usually, however, the second active language reaches a satisfactory standard only after many years of practice and is more suited to interpretation of technical discussions where lexical accuracy is more important than style or very discrete shades of meaning. It is customary only to work into the second active language out of the mother tongue. Bilingual interpreters are much in demand, especially if they can offer a third language.