The series takes its name from the jury panels returned with writs or precepts summoning juries (venire facias juratores) or distraining jurors who had earlier failed to appear in the court when required to do so (distringas juratores). In the early fourteenth century its title was 'Panella Assise'. With the development of litigation by bill it became, from the 1350s to 1440, 'Panella Assise et Bille Finite'. From 1444 to 1465 it was called 'Panella Assise Finite', despite the fact that its contents had by then come to consist mainly of bills and the resulting precepts. From 1474 to the end of its life the file was referred to simply as 'Panella'.
In the earliest years, and despite its title, the file consisted mainly of writs of distringas juratorum in trespass cases and in some appeals and other crown cases; the latter soon disappeared from the file. Non-Middlesex cases which had come to issue were often put to juries in their county of origin by justices of nisi prius sitting in the Lent and summer vacations. The justices of nisi prius were usually either the regular circuit assize justices or a single King's Bench justice. They took with them a record of proceedings in the cases in question, and their clerks endorsed the nisi prius proceedings on the record in an entry called, after its opening word, a 'postea'.
Each individual record was subsequently filed in the Panella file at the beginning of the following term; that is one reason why the files for Easter and Michaelmas terms, following those vacations, are often significantly bulkier than those for the other two terms. Each postea should have been added at the end of the earlier proceedings in the case of the plea roll (KB 27), but may not always have been done. Postea filings continued to be filed among the Panella, in a distinct and bulky section at the back of each file until a date between 1519 and 1522. They were subsequently folded and tied up in county term packets. Most of those packets have broken apart, and the postea filings from 1522 remain unsorted and unlisted.
By the mid fifteenth century the majority of the items on the files were bills of trespass and their associated precepts and jury panels, and it often became bulkier during the terms when the court was in the provinces than it was when it was at Westminster, where it received only a few bills from Middlesex litigants and from privileged persons and officers of the court and their servants who were privileged to sue there; they too were filed with the Panella. Bills referred to King's Bench from Chancery, Parliament and justices of the peace and oyer and terminer were filed among the Recorda (KB 145). In the late fourteenth and early fifteenth centuries ordinary litigants in Middlesex gradually abandoned writs in favour of bills to bring a variety of trespass suits into the Plea Side of King's Bench at Westminster.
These common bills were written in Latin rather than the French used in the earlier petitionary bills to justices in eyre and to Parliament. Once a defendant was in the custody of the marshal of King's Bench, other bills relating to offences committed in any other county could be preferred against him. The number of such bills, now sometimes referred to as custodial bills, grew greatly during the first half of the fifteenth century. Litigants benefited from the faster and cheaper process deriving from the use of precepts to summon defendants rather than original writs issued from Chancery for a fee.
The availability of that type of remedy was extended to litigants from any county by the gradual development of a legal fiction. A common bill of Middlesex would be presented, alleging a fictitious trespass in that county, usually the breaking of a close at Westminster. When the resulting precept to the sheriff of Middlesex to attach the defendant failed to produce him because he was not really a resident there, a judicial writ of latitat was issued to the sheriff of the county where he did live and in which he was deemed to be lurking, to secure his arrest. When the defendant was bailed he was technically in custody, and so a custodial bill, containing the real cause of the complaint, could be brought against him. A judgment of 1452 established that a defendant on bail was answerable to such a bill. Fictitious common bills can often be identified through the name of the filazer, which appears on the precept he issued against the defendant. If it was issued by anyone other than the filazer for London, Middlesex and Kent the bill was clearly fictitious, whereas if it was issued by him it could be a genuine bill of Middlesex. The precise chronology of the development and popularisation of this process has not yet been worked out because the files have not been studied in detail, but it had been established in its essentials before 1500.
By 1503 the common bills and precepts were so bulky that they were removed from the Panella and a new series of Bill Commune Files was created. At that point about 80% of the common bills were fictitious. Between 1519 and 1521 the custodial and privilege bills and their related instruments were also removed from the Panella and a series of Bille Files was created. Similar files had been made up earlier for a few terms on an ad hoc basis, the earliest occasion being in 1489. Following the removal of the bills and posteas between 1503 and 1522 all that remained on the Panella Files were the writs and panels for the few remaining cases brought by original writ. The files consequently became very small, and in a few terms it is known, from notes on the covers of Brevia Regis Files, that no Panella file was made up. The series ceased about 1605.