A review of the common law is not complete without examining equity law, which has its origins in English common law.

Since legal rules cannot be formulated to deal adequately with every possible contingency, applying them mechanically can sometimes result in injustice. To remedy such injustices, the law of equity was developed. The principle of equity is as old as the English common law, but it was hardly needed until the 14th century, since the law until that time was still relatively fluid and informal. As the common law became firmly established, however, its strict rules of proof began to cause hardship. Power to grant relief in situations involving potential injustices lay with the king and the lord chancellor. Eventually the chancellor’s jurisdiction developed into the Court of Chancery, whose function was to administer equity. The chancellor decided each case on its merits and had the right to grant or refuse relief without giving reasons, but common grounds for relief, such as fraud and breach of confidence, came to be recognized. And because the defendant could file an answer, a system of written pleadings developed.

Although a few states in the United States still have separate courts of equity and courts of law, most jurisdictions have merged the procedures of law and equity courts. Although the procedures are merged, the question of whether remedies are legal or equitable is important for determining whether a plaintiff is entitled to a jury trial. Equity law continues to provide unusual and personal remedies for legal disputes of a civil nature. Two of the most familiar equity decrees are the injunction and specific performance. An injunction restrains a party from doing something that would cause irreparable harm if not enjoined or temporarily halted. For example, an injunction may order a manufacturer to stop dumping certain chemicals into a river. Specific performance requires the performance of a duty agreed on in a contract or other agreement.