Mainprise is a concept in English law

English law

English law is the legal system of England and Wales, and is the basis of common law legal systems used in most Commonwealth countries and the United States except Louisiana...

 regarding the release of a prisoner from custody upon sureties

Surety

A surety or guarantee, in finance, is a promise by one party to assume responsibility for the debt obligation of a borrower if that borrower defaults...

 pending trial

Trial

A trial is, in the most general sense, a test, usually a test to see whether something does or does not meet a given standard.It may refer to:*Trial , the presentation of information in a formal setting, usually a court...

. It is nearly identical to the concept of bail

Bail

Traditionally, bail is some form of property deposited or pledged to a court to persuade it to release a suspect from jail, on the understanding that the suspect will return for trial or forfeit the bail...

and has been absorbed into the laws regulating bail in many jurisdictions. When they were distinguished, bail was not restricted in the type of surety upon which a prisoner could be released, while mainprise always involved a specified sum of money. A prisoner released on bail was still technically in custody and could be re-seized, while one delivered on mainprise was no longer in custody and could not have suits brought against him as though he were.