The original four-finger version of the bar was developed after a worker at the Rowntree's factory, York put a suggestion in the recommendation box for a snack that a "man could have in his lunch box for work".  The product was launched in September 1935 in the UK as Rowntree's Chocolate Crisp, and the later two-finger version was launched on May 15, 1936.  Rowntree's Chocolate Crisp was renamed Kit Kat Chocolate Crisp in 1937, and after World War II just Kit Kat....

The original four-finger version of the bar was developed after a worker at the Rowntree's factory, York put a suggestion in the recommendation box for a snack that a "man could have in his lunch box for work".  The product was launched in September 1935 in the UK as Rowntree's Chocolate Crisp, and the later two-finger version was launched on May 15, 1936.  Rowntree's Chocolate Crisp was renamed Kit Kat Chocolate Crisp in 1937, and after World War II just Kit Kat.  The following advertizements ran from 1963-1965 and include the slogan, "Have a break…  have a Kit Kat." Four Beakers 4 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Driver 2 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Switchboard 2 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Four Beakers 2 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Ginger's Friend:  Always keep a cupboard full of Kit Kat (1963) B&W Small Lawnmower:  Always keep a cupboard full of Kit Kat (1963) B&W Small Friend:  Always keep a cupboard full of Kit Kat (1963) B&W Stile:  Always keep a cupboard full of Kit Kat (1963) B&W Lawnmower:  Always keep a cupboard full of Kit Kat (1963) B&W Milkman's Hat:  Have a lunch break, have a Kit Kat (1963) B&W Carpenter's Bench:  Have a lunch break, have a Kit Kat (1963) B&W Breakfast Table:  Have a lunch break, have a Kit Kat (1963) B&W Small Town Square:  Have a lunch break, have a Kit Kat (1963) B&W Small Country Road:  Have a lunch break, have a Kit Kat (1963) B&W Skipping Rope:  Pack a lunch, pack a Kit Kat (1963) B&W Town Square:  Pack a lunch, pack a Kit Kat (1963) B&W Country Road:  Pack a lunch, pack a Kit Kat (1963) B&W Two Women:  Have a break, have a Kit Kat (1963) B&W Pneumatic Drill:  Have a break, have a Kit Kat (1963) B&W Boy's Bike:  Have a break, have a Kit Kat (1963) B&W Covent Garden:  Have a break, have a Kit Kat (1963) B&W Kite:  Have a break, have a Kit Kat (1964) B&W Family Car:  Have a lunch break, have a Kit Kat (1964) B&W Park:  Have a break, have a Kit Kat (1964) B&W Small Kite:  Have a break, have a Kit Kat (1964) B&W Small Park:  Have a break, have a Kit Kat (1964) B&W Close Up:  Have a break, have a Kit Kat (1964) B&W Close Up:  Have a break, have a Kit Kat (1964) B&W Brick Layers:  Pack a lunch, pack a Kit Kat (1964) B&W Train Spotters:  Pack a lunch, pack a Kit Kat (1964) B&W Public Gardens:  Pack a lunch, pack a Kit Kat (1964) B&W Cottage:  Pack a lunch, pack a Kit Kat (1964) B&W Coach Party:  Have a break, have a Kit Kat (1964) B&W Toddlers:  Have a break, have a Kit Kat (1964) B&W Z Cars:  Have a break, have a Kit Kat (1964) B&W Removal Man:  Have a break, have a Kit Kat (1965) B&W Sheep Farmer:  Have a break, have a Kit Kat (1965) B&W Work Bench:  Have a break, have a Kit Kat (1965) B&W Bench:  When you pack a lunch, pack a Kit Kat (1965) B&W Ramble:  When you pack a lunch, pack a Kit Kat.  Look after the Kit Kat (1965) B&W Museum Piece:  There's no eating in here, not even if you were the Queen of Sheba (1965) B&W Music School:  Do try and save your Kit Kat for the break lad (1965) B&W