Fideicommissum

A fideicommissum is a type of bequest in which the beneficiary is encumbered to convey parts of the decedent's estate to someone else. For example, if a father leaves the family house to his firstborn, on condition that she will it to her first child. It was one of the most popular legal institutions in ancient Roman Law for several centuries. The word is a conjunction of the Latin words "fides" (trust) and "committere" (to commit), and thus denotes that something is committed to one's trust.
Exegesis.
This fragment dates to the reign of Caesar Augustus, who first decreed certain requirements for the institution of the "fideicommissum". The institution itself was first mentioned in 200 BC by Terence in "Andria", 290–98: ""tuae mando fide".
It functioned thus: the testator nominated an heir to act as "fiduciarius", entrusted with devising the inheritance to a beneficiary denominated the "fideicommisarius"".
Purpose and enforceability.
The "fideicommissum" enabled the "heres" to be left with the desire of the testator to devise his estate to the "proscripti", and the "heres" would execute the transfer to them.
"Institutes", 2.23.1 indicates that Augustus instituted the "fideicommissum" as legally binding by favouring it in individual cases. Thus its acceptance was directly based on Emperor’s Acts, but even previous to this the "fideicommissum" was always enforceable. The true reason for it lies in the nature of the institution itself. Personal security in Rome was much more important than it is today, primarily because the Romans more greatly valued the duties of friendship. Therefore certain legal institutions were simply premised on "bona fides", e. g. the "tutela", "societas", and "depositum", which did not decrease but rather reinforced their security. Breach of a fiduciary agreement led inevitably to being "infamis", which meant lifetime incapacity of making any commercial actions. This threat was sufficient force to guarantee that the fiduciaries would satisfy their obligations. Being a matter of "honores", consideration was not required.
Applicability.
The great success of the "fideicommissum" as a clever "fraus legi fracta" is proved by reference to its long use; redefined by Justinian I, it may have by his reign existed for 700 years. The reasons lie in the great variety of cases it could resolve and the absence of formal requirements to satisfy.
The practical informality and flexibility of "fideicommissum" is described in "Institutes", 2.23.2. There was no need for a certain formula, any word describing the beneficiaries, such as "rogo", "peto", or "volo", employed with the term ""fidei tuae committo" sufficiently instituted a "fideicommissum". It could be constituted in a will or in a "codicil", made orally or even declared by mere gestures. Most attractive of all, it could be added or revoked or varied after the "institutio heredes" itself. Taking all these advantages in account, it is not surprising that the "legatus", with its strict formal requirements, was far less popular. In later eras, a gradual assimilation of "legates" and "fideicommissa" took place; under Justinian, the two institutions were fused, in an example of the vulgarisation of Roman Law after its classic era.
Restricted "capacitas".
Most important, the "fideicommissum" enabled transferring property by will to those excluded from inheriting. The "lex Voconia" in 169 BC for example did not allow women to be appointed as an heir of Romans listed as wealthy by the "censor". Later, Augustus enforced his "leges Julia" by introducing harsh "leges caducariae", which punished the unmarried and the childless by denying their "capacitas", the privilege of inheriting (the "Lex Julia de Maritandis Ordinibus" was passed in 18 BC and the "lex Papia Poppeia" in 9 AD ("Inst". 1.145)). But the "fideicommissum" enabled a prosperous "pater familias" to appoint his "amicus" as an heir, who would then be trusted with transferring the inherited property to the unmarried "filia". This would ensure her being provided for after his death.
Settlements.
Holding property within a family was of great importance for several reasons. Practically, it ensured the survival of the "familia", its name and wealth. Furthermore it was demanded by the sacral necessity of worshipping the "penates", who would cease to exist if the family did. Most important though, the Roman aristocracy relied on a careful and necessary balance of property, to ensure a balance of power. This could be guaranteed by installing a "fideicommissum". It was able to regulate the succession of several generations by will and hence incorporates the Roman root of family settlements. Unlike "legates", which only allowed passing estate on to a "heres", and "usufructus", which required a determinate person, the "fideicommissum" could be granted to "incertae personae". Using a fideicommissary substitution (making one "fideicommissum" subject to another, enabling the preservation of property within a family for generations through successive trusts) the grantor could therefore direct his "filius" to transfer the estate on to his son at death and so on in perpetuity.
Legacies.
"Legates" are by nature very similar to the "fideicommissum", but the latter did not suffer from the same restrictions. "Legates" could only be charged on a "heres", and the "lex Falcidia" in 41 BC constrained the legacy to three quarters of the entire inheritance. This of course did not apply for the "fideicommissum" and with such could be evaded.
Freeing slaves.
Another existing use of the "fideicommissum" is described in G 2.263-266: "libertas quoque servo per fideicommissum dari potest" if the "heres" or "legatarius" is requested to perform the "manumissio" to become the "patronus" of the slave so freed. 
Freeing slaves was attractive for the "dominus" for several reasons. Firstly, he could get rid of slaves who were of no use to him (because they were thugs, uneducated or incapable of working). Secondly, a freed slave owed the dominus who freed him "honor et sequi", including the procession to the grave. This led to a great number of slaves being freed on the death bed. Rome was gradually crowding with freed slaves, enlarging the poor classes within society. Therefore Augustus passed laws against luxury. They restricted liberation of slaves, but could be partly evaded by the "fideicommissum". The "fideicommissum" also gave way to granting the rights of a "patronus" to the person who was committed to freeing the slave.