The numbers found on plastic items refer to the kind of plastic they are made of. These are useful for recycling purposes. Different recycling programs can only recycle certain numbers and do not accept all the different kinds of plastic.
There are seven numbers that can be shown on plastic containers. Each number corresponds to a specific type of plastic. For example, a common material is polystyrene, or PS. Containers made of PS have the number 6 printed on them, enclosed in a series of arrows. The numbers 1 through 5 correspond to polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, low-density polyethylene and polypropylene, respectively.
The number 7 is printed on materials that contain a combination of plastics. This is an important one to know, because containers made of more than one kind of plastic are often not recycled.