Mancipatio

Mancipatio (f. Latin manus "hand" and capere "to take hold of"), in Roman law, was a verbal contract by which the ownership of certain goods, called res mancipi, was transferred.Res mancipi were goods important in an early agrarian society, like land, rights over land, horses, cattle and slaves. The right of ownership (dominium) for such goods was reserved to Roman citizens (Quirites) and therefore called a "quiritian" or a "quiritary" right.The procedure is described as follows by Gaius: "Mancipatio is effected in the presence of not less than five witnesses, who must be Roman citizens and of the age of puberty, and also in the presence of another person of the same condition, who holds a pair of brazen scales and hence is called Libripens. The purchaser, taking hold of the thing, says: HUNC EGO HOMINEM EX IURE QUIRITIUM MEUM ESSE AIO ISQUE MIHI EMPTUS ESTO HOC AERE AENEAQUE LIBRA (I affirm that this slave is mine according to quiritary right, and he is purchased by me with this piece of bronze and scales). He then strikes the scales with the piece of bronze, and gives it to the seller as a symbol of the price" (Gaius, Institutes, I.119).
See more at Wikipedia.org...

Roman law. The act of transferring things called res mancipi. This is effected in the presence of not less than five witnesses, who must be Roman citizens and of the age of puberty, and also in the presence of another person of the same condition, who holds a pair of brazen scales, and hence is called Libripens. The purchaser (qui mancipio accipit) taking hold of the thing, says I affirm that this slave (homo) is mine, ex jure quiritium, and he is purchased by me with this piece of money (sas) and brazen scales. He then strikes the scales with the piece of money and gives it to the seller as a symbol of the price (quasi pretii loco.) The purchaser or person to whom the mancipatio was made did not acquire the possession of the mancipatio; for the acquisition of possession was a separate act.

Both mancipatio and in jure cessio existed before the twelve tables. Mancipation no longer existed in the code of Justinian, who took away all distinction between res mancipi and nec mancipi.
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.