 Stations may, from time to time, preempt network programs to air special programming of local interest (such as sporting events and coverage of local celebrations); affiliation contracts typically contain restrictions on often this can be done, and may require the displaced programming to be moved to either a sister station or digital subchannel (the latter becoming more common since the transition to digital terrestrial television), or pre-empted to a different time period (such as during the late-night hours) as a make-good.Due to these practices, the major English-language networks do not have a full, centralized schedule (although the major networks do have the ability, at the discretion of affiliates in certain cases, to interrupt programming on all affiliates to broadcast special news reports of national importance), and the types of programming carried outside of network programming (or even the scheduling of network programming, particularly during the daytime) can vary considerably.Due to differing market dynamics, Spanish-language networks such as Telemundo and Univision do have a centralized network schedule with opt-out periods for local programs.