The single subject amendment is a proposed amendment to the United States Constitution that would impose the single-subject rule on federal legislation, limiting the content of bills introduced in Congress to a single subject. The amendment would have the effect of limiting legislative tactics such as logrolling, earmarks, and pork barrel spending. It would also discourage the use of very long omnibus spending bills which are difficult for legislators to read and analyze in the time frame needed for a vote, and to which unrelated riders are often added late in the legislative process. As of 2016, 41 states have single-subject rules in their state constitutions, but the federal Congress has no such rule. Many of these state and local provisions are over a century old, and litigation is often used to enforce the provisions.