Willis : Anglo-American Law 
33 
The elements and characteristics of the right have already 
been considered in connection with personal property of which 
it is a species. 
Contracts is also a generic term, and there are many 
different species of contracts. These species of contracts may 
be classified in different ways. Classified according to 
the nature of the agreement they are unilateral and bilateral, 
and express and inferred. A unilateral contract is one created 
by a promise on one side given for an act other than a promise 
on the other side. A bilateral contract is one created by a 
promise on one side given for a promise on the other side. 
An express contract is one all of whose terms have been 
expressly assented to. An inferred contract is one in which 
either the act of acceptance or some term in the promise is 
inferred as a fact. Classified according to the number of par- 
ties, contracts are joint, several, and joint and several. Classi- 
fied according to formalities, contracts are specialties, written, 
and oral. Classified as to performance, contracts are executed 
and executory, and executory contracts are conditional and 
unconditional. Classified as to validity, contracts are valid, 
voidable, void, and unenforcible. Classified as to subject-matter, 
contracts are principal and accessory. Principal contracts 
include those which affect property rights and those which 
affect personal rights. Those affecting property rights are 
contracts to convey, to lease, to sell, to make a bailment, to 
insure, and to loan. Those affecting personal rights are con- 
tracts to marry and for services, as for example as servant, 
bailee, public calling, profession, agent, etc. Accessory con- 
tracts include suretyship and guaranty, warranty, pledge, 
and mortgage. 
(b) Quasi-Contracts. A quasi-contract is a legal obliga- 
tion (private antecedent legal right in personam) created 
by law because of some unjust enrichment or analogous situa- 
tion. The law imposes such obligations where a benefit has 
been conferred by one upon the other because of request, 
fraud, misrepresentation by one in confidential relation, undue 
influence, duress, and reliance on an unenforcible contract 
or on other legal relations. Other obligations of this sort are 
obligations of statute, custom, and record. The obligation 
requires restitution or its equivalent. 
3—36004 
