The Society of Plastic Industry designed the resin identification coding system to identify plastic type for recycling purposes, these are coded one through seven and printed on the object inside a recycling symbol. The numbers one through six identify specific plastics, and seven is used for miscellaneous.
The SPI code one is used for objects composed of polyethylene terephthalate, PET. It is a polymer resin used in carpets, films and food packaging. SPI code two is used for high-density polyethylene, HDPE, it is made from petroleum and is the most common type of plastic. HPDE is used in bottles, containers, and grocery bags. Another common plastic, polypropylene, is identified through SPI code five. It is used in sheeting, shipping pallets, and auto accessories.