Although Gaius only identifies a single type of contract in re, it is commonly thought that there were four types of these, as Justinian identifies: mutuum (loan for consumption), commodatum (loan for use), depositum (deposit) and pignus (pledge).
Each varied regarding the expected standards of care, transfer of ownership, and other practicalities stemming from the purpose of each.
[1] Real contracts were of limited significance, although they are featured prominently in the works of jurists.
[3] The other three can be distinguished insofar as they are different from traditional concepts of debt from which real contracts developed, are bilateral, do not transfer ownership, are bona fide and praetorian in nature.
[1] It was the oldest contract in re, growing in importance after 326 BC when the lex Poetalia was passed.
As a mutuum did not place on the borrower any specific date on which to return the equivalent thing, this would also be given in a stipulatio if required.
[2] Two exceptions were made, where repayment would be dependent on the success of the operation: the financing of a cargo ship, and the sponsorship of a professional athlete.
[9] A borrower was held (in most juristic texts) to a standard of culpa levis in abstracto – the borrower was liable if his or her conduct fell short of the diligentia (care) of a bonus paterfamilias – a good, respected, head of the family.
Custodia was a form of strict liability, where the only situation when the borrower would not be liable would be actions of a "greater force" (vis maior) such a theft with force, or what is called in the modern English law an act of God.
[12] If the borrower was liable, then he had an action available against the thief (the actio furti) or damager under the Lex Aquilia.
This meant that the depositee was liable if found to have been grossly negligent: careless to the extent that bad faith could almost be assumed.
The depositee was also bound to hand over any accretions of the thing, such as any young born to a deposited animal.
Originally, it seems that the expenses could be set against the value of the thing in the actio depositi, but, if so, this ability came to an end in the time of Justinian.
In contrast to a usual deposit, ownership did pass, granting more rights to the depositee to protect the property.
Fiducia remained popular with lenders who enjoyed increased security, whereas a pignus was more useful to the pledger.
[16] The pledgee, if he was in physical control of the object (as was usually the case) was required to safeguard the thing.
[16] Like the borrower, a pledgee was held to the culpa levis in abstracto standard; again, this may have developed from custodia.
[17] The pledgor was liable for damage done by his thing deposited, if he acted without the care of a bonus paterfamilias.
If the pledgee fell short of the expected standard, the contract was terminated immediately.