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Regardless of the knowledge skill you’re using, refer to the rules here when making a knowledge check. Another use of the knowledge skills is for Monster Knowledge Checks.

Common Knowledge: This includes the kind of general information that is commonly known about a given topic.

Expert Knowledge: This includes the kind of specialized information that only an expert in the field of study could possibly know.

Master Knowledge: This includes the kind of esoteric information that only a master in the field of study could possibly know.

Check

Before errata, the DCs were originally the following:

Level of Knowledge DC
Common 15
Expert 20
Master 25
Paragon tier +10
Epic tier +15

Knowledge Skills