Codex is a game show that aired on Channel 4 from 12 November 2006 to 15 December 2007 and was hosted by Tony Robinson.
If they succeed then two of the symbols of the codex are replaced with the two letters they represent; this is shown to the viewers but not to the team until later in the programme.
At the end of the show the four people eliminated in the head-to-heads work together to try and decode the codex using the revealed letters.
The surviving team member then enters the museum's Reading Room where they are presented with five more artefacts.
Sometimes the clue can be cryptic - for example, in one programme the term "I'm buttercup" referred to a vessel (cup) in the form of a male sheep (ram being an animal that's a 'butter' - i.e. butts things).