All states have at least one; larger states have several,
divided by geographic district within the state (e.g.,
the Western District of Pennsylvania).
Many issues before federal courts deal with application
and interpretation of federal legislation. Often the circuits
may diverge in matters of their interpretation. Circuits
frequently look to each other’s opinions as persuasive
authority and often consensus will emerge; if not, the
U.S. Supreme Court may grant certiorari to resolve a split
in the circuits.
Federal courts may hear cases on state law issues under
diversity jurisdiction. In such cases, the federal courts
look to opinions of the highest court of the state as
mandatory authority.