What is Bill Of Middlesex?

Legal Definition
The Bill of Middlesex was a legal fiction used by the Court of King's Bench to gain jurisdiction over cases traditionally in the remit of the Court of Common Pleas. Hinging on the King's Bench's remaining criminal jurisdiction over the county of Middlesex, the Bill allowed it to take cases traditionally in the remit of other common law courts by claiming that the defendant had committed trespass in Middlesex. Once the defendant was in custody, the trespass complaint would be quietly dropped and other complaints (such as debt or detinue) would be substituted. Part of a large reform movement to prevent equitable courts such as the Court of Chancery undermining their business, the Bill was far cheaper and faster than the older equivalents used by the Chancery and Common Pleas, leading to a drop in their business and an increase in that of the King's Bench. As such, the Chancery issued injunctions in an ineffective attempt to prevent its use. The Bill was finally abolished by the Uniformity of Process Act 1832.

As a result of reforming actions such as the Bill of Middlesex, the Common Pleas became increasingly conservative and resistant to King's Bench changes because of the impact they had on the business of the Common Pleas. This was best emphasised by Slade's Case, a struggle between the old and new forms of suing for breach of contract; although an equilibrium between the common law courts was finally reached, it eventually led to their dissolution with the Supreme Court of Judicature Act 1873, and merger into a single High Court of Justice.
-- Wikipedia
Legal Definition
An old form of process similar to a capms, issued out of the court of king's bench in personal actions, directed to the sheriff of the county of Middlesex, (hence the name,) and commanding him to take the defendant and have him before the king at Westminster on a day named, to answer the plaintiff's complaint. State v. Mathews, 2 Brev (S. O.) 83: Sims v. Alderson, 8 Leigh (Va.) 484. 3. A formal written petition to a superior court for action to be taken in a cause already determined, or a record or certified account of the proceedings in such action or some portion thereof, accompanying such a petition.
-- Black's Law Dictionary
Legal Definition
A kind of capias directing the sheriff to have the defendant in court on a certain day to answer an action of trespass. See 8 Leigh (Va.), 479.
-- Ballentine's Law Dictionary