Rangers and Celtic have been nicknamed "The Old Firm" due to the commercial benefits generated by their rivalry. The nickname seems to have sprung up in the early 1900s when matches between them were drawing crowds of 60-70,000 on a regular basis.
The 1909 Scottish Cup Final ended in a riot, not, as is generally supposed, due to sectarian rivalry, but because the crowd thought the result had been fixed, the first match having ended in 2-2 draw, and the replay in 1-1 draw, meaning a third game was now necessary. The previous season, the Glasgow Cup Final had run for three matches and there had been suspicions at that time, that these games were being manipulated.