Faithfulness constraints (in Optimality Theory) encode the tendency to change as little as possible to the input: it is best (e.g. for word recognition) not to insert any segment, not to delete any segment, not to switch segments around, not to assimilate, not to coalesce, etc. If this does happen, there must be a markedness constraint triggering this. Related to correpondence, e.g. base-reduplicant correspondence.