CECIL is a random number generator used to set three-digit target numbers for numbers rounds on Countdown. CECIL is an acronym, short for "Countdown's Electronic Calculator in Leeds", though CECIL in fact does not perform calculations. Although production of Countdown moved to Manchester in 2009, the name CECIL is still used today. In the first episode of Countdown, presenter Richard Whiteley stated that the name CECIL was an affectionate nod to ERNIE, the random number generator used to generate Premium Bonds winners. The choice of name is a tribute to the late Cecil Korer, former Head of Entertainment at Channel 4, who first commissioned Countdown in 1982.
Before each numbers round begins, CECIL is reset and displays "000". During the round, a co-presenter – originally Beverley Isherwood, later Carol Vorderman and most recently Rachel Riley – places six numbered tiles into recessed boxes directly below CECIL's three-digit display. To prompt CECIL to generate a random target number, which the contestants must reach using the numbers on the tiles, the co-presenter then presses a button. CECIL's digits then briefly flash at random, before settling on a three-digit number which is declared the target figure. The target numbers generated by CECIL were originally 100 to 999 inclusive, but the lowest possible target was later revised to 101. This lower limit was used for an indeterminate period of time before a 100 target reappeared on the "new CECIL" in Series 75 in 2016. A 100 tile is one of the four large number tiles used in numbers rounds, and therefore the target could potentially be reached simply by declaring a number, without needing to perform any calculations (which notably happened for the very first time in 8 Out of 10 Cats Does Countdown Series 22, Episode 3 in January 2022).
CECIL's appearance has varied throughout Countdown history to match the set design of the time. However, the functionally of its surroundings has remained unchanged. Beneath its display and the aforementioned six recesses is a sheet of card, on which the arithmetician writes down solutions to the numbers rounds. Collectively this is known as "the numbers board". Throughout most of Countdown's history, the numbers board has been integrated into a wall of the main studio set. For Championship of Champions XI in 2003, a new striped set was introduced which moved the numbers board to a separate island isolated from the main desk. The current set design, introduced when Jeff Stelling became the main presenter of Countdown, has CECIL mounted directly behind the board used for letters rounds.
On 21 January 2013 (during the 30th Birthday Championship), with the advent of Countdown being produced in high definition, CECIL's display was upgraded from a segmented LED display to a flat-screen monitor, as were the displays used to show the contestants' scores. A further change was made to the displays when the "new" blue-and-brown set was introduced at the beginning of Series 77 in July 2017. The blue colour of the displays was brightened slightly, and CECIL now cycles through random numbers for a notably shorter amount of time before landing on a target.
On 28 May 2019 (during Series 80), CECIL debuted with a permanent whiteboard, marking the end of 36.5 years of continuous usage of paper clipped to a board.
Though little more than a circuit board, on occasion CECIL has seemed to develop a mind of its own. In particular, CECIL is noted for a lack of sense of occasion and has a tendency to offer incongruously easy targets at crucial moments: infamous examples of this include two extremely easy targets in the final of the Supreme Championship, as well as Rachel Riley's first ever numbers game as arithmetician (pictured below).