C Series is the third series of QI; all of its episodes involve topics beginning with "C". It began on 30 September 2005 and ended on 16 December 2005. It was released on DVD on 1 September 2008; on 4 September 2011, it was also featured as part of "Complete Series (A-C)". The episodes were named retroactively.
This series is the only one which doesn't feature a Christmas special, perhaps ironically, given the letter theme of the series. It featured several task episodes, including a couple involving "Spot the __" themes (maybe a precursor to Elephant in the Room). This series features the first forfeit given to the audience and the first appearance of David Mitchell, who later became the seventh-most common panellist on the show.