In common law, a writ is a formal written order issued by a body with administrative or judicial jurisdiction; in modern usage, this body is generally a court. Warrants, prerogative writs, subpoenas, and certiorari are common types of writs, but many forms exist and have existed.

In its earliest form, a writ was simply a written order made by the English monarch to a specified person to undertake a specified action; for example, in the feudal era, a military summons by the king to one of his tenants-in-chief to appear dressed for battle with retinue at a specific place and time. An early usage survives in the United Kingdom, Canada, Australia, and some other Commonwealth countries in a writ of election, which is a written order issued on behalf of the monarch (in Canada, by the Governor General and, in Australia, by the Governor-General for elections for the House of Representatives, or state governors for state elections) to local officials to hold a general election. Writs were used by the medieval English kings to summon people to Parliament (then consisting primarily of the House of Lords) whose advice was considered valuable or who were particularly influential, and who were thereby deemed to have been created "barons by writ".

Sometime before the tenth century, officials in England began utilizing writs to convey orders. A "writ" was simply a short written command issued by a person in authority. It was customary for the sender to seal such a command as proof of its authenticity. In the days when writing was a rare art, a writ was revered because the person receiving the command was unlikely to deny or question its legitimacy. The Norman Conquest of England in 1066 led to the establishment of a strong, centralized monarchy. The first Norman King of England, William the Conqueror, modified writs to become mainly framed in Latin, increased the number of writs to cover additional royal commands, and established the Curia Regis in England. The Curia Regis, a Latin term meaning "royal council", consisted of the King of England and his loyal advisors. The Curia Regis accompanied the King as he travelled. This council administered all of the King's governmental activities, including judicial matters.

One of the most important members of the Curia Regis was the Lord Chancellor. The Lord Chancellor led the chancery. Chancery is a general term for a medieval writing office that was responsible for the production of official documents. The Lord Chancellor wrote writs on behalf of the King, maintained all official documents, and acted as the keeper of the royal seal. This position, in effect, placed the Lord Chancellor as the head of the English legal system. The King, however, was the ultimate leader of the kingdom; therefore, the Lord Chancellor issued writs under the guidance of what he believed to be in the best interests of the King. Between the twelfth and thirteenth centuries, the Lord Chancellor had a large control over the issuance of all original writs. In this history of English common law, original writs began a legal proceeding, while a judicial writ was issued during a legal proceeding.

The writ was a unique development of the Anglo-Saxon monarchy and consisted of a brief administrative order, authenticated (innovatively) by a seal. Written in the vernacular, they generally made a land grant or conveyed instructions to a local court. In the beginning, writs were the documents issued by the King's Chancellor against a landowner whose vassal complained to the King about an injustice, after a first summon by the sheriff to comply had been deemed fruitless. William the Conqueror took over the system unchanged, but was to extend it in two ways: first, writs became mainly framed in Latin, not Anglo-Saxon; second, they covered an increasing range of royal commands and decisions. Writs of instruction continued to develop under his immediate successors, but it was not until Henry II that writs became available for purchase by private individuals seeking justice, thus initiating a vast expansion in their role within the common law.

Writs could take two main forms: 'letters patent', which were open for all to read, and 'letters close' for one or more specified individuals alone.

The development of writs as a means of commencing a court action was a form of "off-the-shelf" justice designed to enable the English law courts to rapidly process lawsuits by allocating each complaint form into a standard category that could be dealt with by standard procedures. The complainant applied to the court for the writ most relevant to his complaint to be sent to the wrongdoer, which ordered him under royal authority to attend a royal court to answer for his actions. The development was part of the establishment of a Court of Common Pleas, for dealing with commonly made complaints by subjects of the crown, for example: "someone has damaged my property". The previous system of justice at the royal court of Chancery was tailor-made to suit each case and was thus highly time-consuming. Thus eventually the obtaining of a writ became necessary, in most cases, to have a case heard in one of the Royal Courts, such as the King's Bench or Common Pleas. Some franchise courts, especially in the Counties Palatine, had their own system of writs, which often reflected or anticipated the common law writs. The writ was "served" on (delivered in person to) the wrongdoer and acted as a command that he should appear at a specified time and date before the court specified in the writ, or it might command some other act on the part of the recipient.

Where a plaintiff wished to have a case heard by a local court or by the justice of an Eyre if one happened to be visiting the county, there would be no need to obtain a writ. An informal complaint could usually start actions in local courts. However, if a plaintiff wished to avail himself of Royal — and by implication superior — justice in one of the King's courts, then he would need a writ, a command of the King, to enable him to do this. Initially, for common law, recourse to the King's courts was unusual, and something for which a plaintiff would have to pay. For most Royal Courts, the writ would usually have been purchased from the Chancery, although the court of the Exchequer, being, in essence, another government department, could issue its own writs.