A public good has two defining properties: It is non-rivalrous: consumption of the good by one party does not reduce the ability of any other party to consume that good. It is non-excludable: it is not possible to prevent anyone from consuming it. The opposite of a public good is a private good. A private good has the following properties: Consumption of it is rivalrous: in as much as one person uses or consumes it, another person cannot use or consume (...) it. It is excludable: it is possible to exclude people from using it. The difference between private and public goods has implications for making decisions about who pays for them. The rivalrous property of a private good makes it desirable that the consumer of the good should be the one to pay for it; the excludable property of a private good makes it possible to charge the consumer for consuming it – if you don't pay then you will be excluded from having or using it. (shrink)