In common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

, a writ is a formal written order issued by a body with administrative or judicial jurisdiction

Jurisdiction

Jurisdiction is the practical authority granted to a formally constituted legal body or to a political leader to deal with and make pronouncements on legal matters and, by implication, to administer justice within a defined area of responsibility...

; in modern usage, this body is generally a court

Court

A court is a form of tribunal, often a governmental institution, with the authority to adjudicate legal disputes between parties and carry out the administration of justice in civil, criminal, and administrative matters in accordance with the rule of law...

. Warrants, prerogative writ

Prerogative writ

Prerogative writs are a class of writs which originate from English law. Originally they were available only to the Crown, but later they were made available to the monarch's subjects through the courts.The prerogative writs are:*certiorari...

s and subpoena

Subpoena

A subpoena is a writ by a government agency, most often a court, that has authority to compel testimony by a witness or production of evidence under a penalty for failure. There are two common types of subpoena:...

s are common types of writs but there are many others.
Originally, a writ became necessary, in most cases, to have a case heard in one of the Royal Courts, such as the King's Bench

King's Bench

The Queen's Bench is the superior court in a number of jurisdictions within some of the Commonwealth realms...

 or Common Pleas

Court of Common Pleas (England)

The Court of Common Pleas, or Common Bench, was a common law court in the English legal system that covered "common pleas"; actions between subject and subject, which did not concern the king. Created in the late 12th to early 13th century after splitting from the Exchequer of Pleas, the Common...

. Some franchise courts, especially in the Counties Palatine

County palatine

A county palatine or palatinate is an area ruled by an hereditary nobleman possessing special authority and autonomy from the rest of a kingdom or empire. The name derives from the Latin adjective palatinus, "relating to the palace", from the noun palatium, "palace"...

, had their own system of writs that often reflected or anticipated the common law writs.