Logical difference is an important notion to distinct different versions of knowledge-base systems. To capture such difference in terms of logic consequence, clause consequence and prime clause consequence respectively, this paper proposes three notions of difference over relevant signatures — logical difference, clausal difference and prime difference. They are closely related to forgetting. It is generally intractable to compute such differences, even for Horn theories. Preliminary experimental results on clausal difference and prime difference illustrate an interesting phase transition phenomenon over random 3-CNF theories.