American and Canadian football are gridiron codes of football that are very similar. Both have their origins in rugby football. There are, however, some key differences.
Football was introduced to North America in Canada by the British Army garrison in Montreal, which played a series of games with McGill University. In 1874, USA's Harvard University hosted Canada's McGill University to play the new game derived from rugby football in a home-and-home series. When the Canadians arrived several days early, to take advantage of the trip to see Boston and the surrounding areas, they held daily practices. During this time, the Americans were surprised to see the Canadians kick, chase, and then run with the ball. Picking up and running with the ball violated a basic rule of the American game of the day; when the US captain (Henry Grant) pointed this out to the captain of the Canadian team (David Roger), the reply was simple: Running with the ball is a core part of the Canadian game. When the American asked which game the Canadians played, David replied "rugby". After some negotiation, it was decided to play a game with half and half Canadian/US rules. Thus, many of the similarities and differences between the Canadian and American games indeed came out of this original series where each home team set the rules. For instance, Harvard, because of a lack of campus space, did not have a full-sized rugby pitch. Their pitch was only 100 yd (91 m) long by 50 yd (46 m) wide with undersized endzones (slightly less than the 53⅓-yard width of the current regulation-sized field for American football). Because of the reduced field, the Harvard team opted for 11 players per side, four fewer than the regulation 15 of Rugby Union. To generate more offense, Harvard also increased the number of downs from three, as set by McGill, to four. Furthermore, the Harvard players so enjoyed running with the ball, this rule was wholly adopted into all Harvard play following the two games with McGill. While the American team bested the Canadian (3-0 and a following tie game), both countries' flavours of football were forever changed and linked to one another. Both the Canadian and American games still have some things in common with the two varieties of rugby, especially rugby league, and because of the similarities, the National Football League (NFL) had a formal relationship with the Canadian Football League (CFL) between 1997 and 2008.