The SPI resin identification coding system is a set of symbols placed on plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

s to identify the polymer type. It was developed by the Society of the Plastics Industry

Society of the Plastics Industry

Founded in 1937, The Society of the Plastics Industry, Inc. is the trade association representing one of the largest manufacturing industries in the United States. SPIs members represent the entire plastics industry supply chain in the US, including processors, machinery and equipment manufacturers...

 (SPI) in 1988, and is used internationally.
The primary purpose of the codes is to allow efficient separation of different polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 types for recycling

Recycling

Recycling is processing used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution and water pollution by reducing the need for "conventional" waste disposal, and lower greenhouse...

.

The symbols used in the code consist of arrows that cycle clockwise to form a rounded triangle and enclosing a number, often with an acronym representing the plastic below the triangle.