§11. What turns leases for uncertain terms into leases from year to year, is the landlord's reserving annual rent. A tenant placed on land without any terms prescribed or rent reserved, is strictly a tenant at will; and it has been held that such tenant is not entitled to notice to quit; but the general rule now seems to be, that even in such case the six months' notice is necessary; or, as in some states, a reasonable notice. §12. An _estate at sufferance_ is that which is acquired by a tenant who has come into lawful possession of land, but who holds over by wrong after his interest has ceased. He is not entitled to notice to quit; and where there is no special statute, he is not liable for rent; and the landlord may enter, and remove the tenant and his goods with such gentle force as may be necessary. If undue force is used, the landlord would be liable to an action for forcible entry and detainer. Chapter LIV. Contracts in General. §1. A _contract_ is an agreement between two or more persons, by which the parties agree to do, or not to do, a particular thing. Contracts are _executory_, when the stipulations remain to be executed, or when one party agrees to sell and deliver, at a future time, for a stipulated price, and the other agrees to accept and pay. Contracts are _express_, when the parties contract in express words, or by writing; _implied_, when an act has been done which shows that the parties must have intended to contract; as, when a person employs another to do some service, it is presumed that the party employing intended to pay for the labor performed. §2. Contracts are also distinguished as specialties and simple contracts. A _specialty_ is a contract under seal; as a deed, or a bond. But we shall here consider chiefly that common class of contracts called _simple contracts_, or _contracts by parol_. _Parol_ signifies by word of mouth. Applied to contracts, however, it not only means verbal contracts, but includes written contracts not under seal. Both are simple contracts; the distinction between them is in the mode of proof. The mutual understanding of the parties to a verbal contract may be proved by parol evidence. But as the real intention of parties is more likely to be expressed in a written contract, the rule of law is, that parol evidence may not be admitted to contradict or vary the terms of a written instrument. It may however be admitted to explain what is doubtful, or to supply some deficiency.