States must honor the legality of one another's civil laws because of the:

States must honor the legality of one another's civil laws because of the Full Faith and Credit Clause

The Full Faith and Credit clause outlines the duties that states within the U. S owe each other. They are required to respect, the public acts, records, and judicial proceedings of every other state.

Full Faith and Credit Clause

The clause addresses the duties that states within the United States have to respect, that is the "public acts, records, and judicial proceedings of every other state. According to the Supreme Court, there is a difference between the credit owed to laws as compared to the credit owed to judgments.