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, McGill invited Harvard to Montreal to play their new game derived from Rugby football in a home and home series. Many of the similarities and differences between the Canadian and American games indeed came out of this original home and home series where each home team set the rules. For instance, Harvard due to lack of campus space did not have a full-sized rugby pitch. Their pitch was only 100 yards long x 50 yards wide with undersized endzones (still the regulation size for American Football). Because of the reduced field, the Harvard team opted for 11 players per side, four less than the regulation 15 of Rugby Union (13 in Rugby League). The number of downs was also increased by Harvard to 4 from 3 set by McGill to generate more offence. 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 has established a formal relationship with the Canadian Football League.
Many, perhaps most, of the rules differences have arisen because of rules changes in American football in the early twentieth century which have not been copied by Canadian football. The major Canadian codes never abolished the onside scrimmage kick (see Kicker advancing the ball below) or restricted backfield motion, while the NCAA (from whose code all American codes derive) did. Canadian football was late in adopting the hand snap and the forward pass, although one would not suspect the latter from play today. Additionally, Canadian football was slower in removing restrictions on blocking, but caught up by the 1970s so that no significant differences remain today. Similarly, differences in scoring (the Canadian game valuing touchdowns less) opened up from the late 19th century but were erased by the 1950s. For these reasons, this article would have been considerably longer during about 1910-50. An area in which American football has been more conservative is the retention of the fair catch (see below).
In some regions along the Canada-USA border, especially western areas, some high schools from opposite sides of the border will regularly play games against one another (typically one or two per team per season). By agreement between the governing bodies involved, the field of the home team is considered a legal field, although it is a different size from one school's normal field. In all but a few cases, the rules of the home team are followed throughout the game.
Because of the similarities between the two games, many outside of Canada today consider Canadian football a minor variation of the American game and the CFL to be a minor league (which is a misconception) and not a major professional league. Indeed, many Canadian Football League players are Americans who grew up playing American football. CFL games are even broadcast in the United States on regional cable sports networks covering large portions of the US, though media coverage is generally of a much lower level than that of the NFL.
However, the CFL is popular in Canada per an Association for Canadian Studies survey (PDF file). Another survey found that the CFL was the second most popular league in Canada, after the NHL, and before the NFL.
For individuals who played both American and Canadian football professionally, their career statistic totals are considered to be their combined totals from their careers in both the CFL and NFL. Warren Moon, for example, was the all-time professional football leader in passing yards after an illustrious career in both leagues. He was surpassed in 2006 by Damon Allen, whose career has been exclusively in the CFL.