Education, Key qualification
Explanation of the language combinations:
A Language
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.
B Language
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 into a "B" language in only one of the two modes of interpretation.
C Language
The language(s) of which the interpreter has a complete understanding and from which she or he works. Interpreters often have several C languages.