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 1962-1963 and include the slogan, "Have a break…  have a Kit Kat." Truck Repair:  Have a break, have a Kit Kat (1962) B&W Tea Tray:  Have a break, have a Kit Kat (1962) B&W Tea Tray Canteen:  Have a break, have a Kit Kat ( 1962) B&W Tea Tray Housewife:  Have a break, have a Kit Kat (1962) B&W Tea Tray Café:  Have a break, have a Kit Kat (1962) B&W Tea Tray Office:  Have a break, have a Kit Kat (1962) B&W Carpentry:  Have a break, have a Kit Kat (1962) B&W Clerk:  Have a break, have a Kit Kat (1962) B&W Booking Clerk:  Have a break, have a Kit Kat (1962) B&W Built Up Pack Shot:  Have a break, have a Kit Kat (1963) B&W Cake Maker:  Have a break, have a Kit Kat (1963) B&W Small Tea Tray:  Have a break, have a Kit Kat (1963) B&W Small Canteen:  Have a break, have a Kit Kat (1963) B&W Small Morning Tray:  Have a break, have a Kit Kat (1963) B&W Flowers:  Have a break, have a Kit Kat (1963) B&W Road Map:  Have a break, have a Kit Kat (1963) B&W Café Table:  Have a break, have a Kit Kat (1963) B&W Small Halt:  Have a break, have a Kit Kat (1963) B&W Small Fishing:  Have a break, have a Kit Kat (1963) B&W Flowers Revealing:  Have a break, have a Kit Kat (1963) B&W Roadside Halt:  Have a break, have a Kit Kat (1963) B&W Beach:  Have a break, have a Kit Kat (1963) B&W Fishing:  Have a break, have a Kit Kat (1963) B&W Boating:  Have a break, have a Kit Kat (1963) B&W Cricket:  Have a break, have a Kit Kat (1963) B&W Small Beach:  Have a break, have a Kit Kat (1963) B&W Small Boat:  Have a break, have a Kit Kat  (1963) B&W Small Cricket:  Have a break, have a Kit Kat (1963) B&W Tractor:  Have a break, have a Kit Kat (1963) B&W Tug:  Have a break, have a Kit Kat (1963) B&W Small Tug:  Have a break, have a Kit Kat (1963) B&W Small Desk:  Have a break, have a Kit Kat (1963) B&W Small Kitchen:  Have a break, have a Kit Kat (1963) B&W GPO Sorting Office:  Have a break, have a Kit Kat (1963) B&W Tea Caddy:  Have a break, have a Kit Kat (1963) B&W Desk:  Have a break, have a Kit Kat (1963) B&W Ginger 4 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Market Research:  Always keep a cupboard full of Kit Kat (1963) B&W Visitor:  Always keep a cupboard full of Kit Kat (1963) B&W Ginger 2 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Driver 4 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W Switchboard 4 Fingers:  Always keep a cupboard full of Kit Kat (1963) B&W