In simultaneous interpretation, this refers to interpreting from an interpretation, not directly from the speaker, and it is used when an interpreter does not know the language of the speaker.

This is how it works: The first interpreter interprets into his target language. The second interpreter listens to the first interpreter and interprets into her target language. Click on the link to see a drawing of a common relay configurations.

Relay harms quality, increases the risk of errors, and slows down the interpretation. For this reason, except in the case of rare languages, simultaneous interpreters hired for a meeting or convention must be able to interpret from all of its source languages.