The name Old Firm is a collective term for the Glaswegianfootball clubs, Rangers and Celtic. It is derived from the profitable nature of contests between the two.
Games between these two biggest clubs in Scotland (and among the biggest in the United Kingdom) are characterised by sectarian posturing from both sets of fans, which can and often does spill over into violence. Celtic represent the Scottish Catholic community and Rangers the Protestant community. Old Firm ties (matches) have been known to be a flashpoint for sectarian strife in Northern Ireland.