De feudis theses LVI

This is commonly divided into proper and improper: the former is called the actual delivery of the vassal into possession of the feudal thing; the latter is accomplished by some bodily act, and usually by these words: I invest you with this sword, ring, or shield.

XXXVIII.
The oath of fealty, which must be performed by the vassal after investiture, contains six things principally within itself. For he shall swear to his lord to be harmless, safe, honest, useful, easy, and possible.

XXXIX.
Or it can also be performed generally, that is, that he wishes to show himself faithful and obedient to the lord: which form is mostly received in use today.

XL.
The effects of fiefs are various. For first, a certain right of using and enjoying is acquired for the vassal, which others call "useful ownership" dominium utile, but we call it "quasi-ownership," while the direct and true ownership remains with the grantor.

XLI.
The second effect consists in the action which descends from this contract, where because of a mutual synallagma contractual exchange one is obligated to the other for faith, defense, liberation, preservation of mutual law, fulfillment of promises, and eviction; and this is sometimes called an action from a fief, sometimes an action from stipulation, sometimes a useful claim of a thing.

XLII.
Thirdly, it pertains to the effect that neither can alienate the fief without the consent of the other, under penalty of loss of his own right.