Complex sentence: Association football was first codified in the 1860s in the south of London by teams of upper-class former public school pupils and Oxbridge graduates, but by the following decade the game had spread to the industrial towns of the north.
Split into simpler sentences: Association football was first codified in the 1860s in the south of England and played by teams of upper-class former public school pupils and Oxbridge graduates. By the following decade, however, the game had spread to the industrial towns of the north.