In most cases, conference interpreters work from their passive languages into their mother tongue. The mother tongue is their active language and is sometimes known as the A-language.
Some interpreters have an excellent command of a language other than the native language and are able to work into that language from one or more of their other languages. They have a second active language. An interpreter who is able to work from his or her mother tongue into a second active language is said to do a retour. Some interpreters who have a retour language work into that language only in consecutive, not in simultaneous.
Passive languages are languages the interpreter fully understands and would most often speak to some extent, only not enough to work into that language.
A : The interpreter’s native language (or another language strictly equivalent to a native language), into which the interpreter works from all her or his other languages in both modes of interpretation, simultaneous and consecutive. All members must have at least one ‘A’ language but may have more than one.
B : A language other than the interpreter’s native language, of which she or he has a perfect command and into which she or he works from one or more of her or his other languages. Some interpreters work into a ‘B’ language in only one of the two modes of interpretation.
C : Languages, of which the interpreter has a complete understanding and from which she or he works.