A Co-event is an amateur tournament similar to a Scrabble or chess tournament. Players meet up with their Countdown board games (or in some cases, suitable computer software) and play a number of games of Countdown, usually 5 or 6, and the player with the most wins is the tournament winner. In the event of a tie, the number of points scored is used to separate them.
There are two distinct types of tournament: head-to-head "Edinburgh-style" tournaments, and "Bristol-style" tournaments where everyone in the room plays from the same set of letters and numbers. The Countdown in Lincoln tournaments use head-to-head games, i.e. each 'table' of three players selects its own separate letters and numbers (one person acting as host, 'Dictionary Corner' and 'Rachel' whilst the other two play each other), while the Countdown in Bristol tournament uses the second system whereby players play in pairs, but all players have the same letters, numbers and conundrum (usually displayed on a board at the front of the room).
This second system eliminates the luck factor as no player or players have "better" selections than anyone else over the course of the day. However, some dislike it because, since they cannot select the letters or numbers themselves, it denies them the chance to use tactical selections to win individual games, for example picking an unusual numbers selection against an opponent known to be weak at the numbers game.
Some otherwise most Bristol-style tournaments have a system for numbers games that compromises between the two formats: A contestant from each pair makes a choice of numbers, just as on the show or in the Edinburgh format and, once everybody has chosen, a selection is revealed for each possible number of large and small numbers in one way or another, the 1st to introduce this was .co.mk in 2015.
Since 2016, the majority of tournaments have been run as part of organisational body FOCAL.