The most common recycling symbols are SPI symbols, which consist of a single digit between one and seven placed in the middle of three arrows arranged clockwise into a triangular shape. The SPI resin identification coding system is an internationally recognized system that enables more efficient recycling of plastic products.
The Society of the Plastics Industry developed the SPI resin identification coding system in 1988. Each symbol's number corresponds to a different type of polymer used to manufacture the product. One of the most common SPI symbols is the number one contained within three arrows. This design corresponds to items made with polyethylene terephthalate, such as soft drink bottles, carpet and polyester fibers. Another SPI number often encountered in everyday life is two, which corresponds to high-density polyethylene products.