House Rule
A House Rule is any game rule that doesn't follow the official rules provided by Ars Magica Fifth Edition. ArM5 encourages you to make house rules to better fit the game to your preferred style and preferences.
Sagas are not expected to employ all the rules provided in the ArM5 line, and those that don't include a certain supplement are not considered to be making a House Rule when they arbitrate issues regarding it using the core rules. However, if they go beyond the core rules or specifically alter how they treat something to not conform to a supplement they use, they are making a house rule.
Typical areas of house ruling include Magic Resistance and the system of Combat. Some sagas also do not utilize the initaition rules provided for Mysteries, ignore the Spell Guidelines, or so on. Of course, you ca play perfectly well with no house rules. House rules represent more the troupe's choice of style and emphasis than any flaw in the system.
See Also
Legacy Page
The history of this page before August 6, 2010 is archived at Legacy:house_rule