A more thorough explanation:

liber niger

Ancient records, such as the register in the Exchequer and the register of charters of abbeys and cathedrals, are called liber niger. The term comes from the Latin words "black book."

• The register in the Exchequer is an example of a liber niger.
• The register of charters of abbeys and cathedrals is another example of a liber niger.

These examples illustrate how liber niger refers to ancient records that were kept in the Exchequer and in abbeys and cathedrals. These records were important for keeping track of financial transactions and legal documents.