A clef (from French: "key") is a musical symbol used to indicate the pitch of written notes. Placed on one of the lines at the beginning of the stave, it indicates the name and pitch of the notes on that line. This line serves as a reference point by which the names of the notes on any other line or space of the stave may be determined. Only one clef that references a note in a space rather than on a line has ever been used.

The Conference and Labs of the Evaluation Forum (formerly Cross-Language Evaluation Forum), or CLEF, is an organization promoting research in multilingual information access (currently focusing on European languages). Its specific functions are to maintain an underlying framework for testing information retrieval systems and to create repositories of data for researchers to use in developing comparable standards. The organization holds a forum meeting every September in Europe. Prior to each forum, participants receive a set of challenge tasks. The tasks are designed to test various aspects of information retrieval systems and encourage their development. Groups of researchers propose and organize campaigns to satisfy those tasks. The results are used as benchmarks for the state of the art in the specific areas.,