Recycling by product massive number of types of plastic, they each carry a resin identification code, and must be sorted before they can be recycled. This can be costly; while metals can be sorted using electromagnets, no such 'easy sorting' capability exists for plastics. In addition to this, while labels do not need to be removed from bottles for recycling, lids are often made from a different kind of non-recyclable plastic. To help in identifying the materials in various plastic items, resin identification code numbers 1-6 have been assigned to six common kinds of recyclable plastic resins, with the number 7 indicating any
Recycling symbol assist recyclers with sorting the collected materials, but they do not necessarily mean that the product/packaging can be recycled either through domestic curbside collection or industrial collections. The SPI symbols are loosely based on the Möbius loop symbol, but feature simpler bent (rather than folded over) arrows that can be embossed on plastic surfaces without loss of detail. The arrows are formed into a flat, two-dimensional triangle rather than the pseudo-three-dimensional triangle used in the original recycling logo. The different resin identification codes can be represented by Unicode icons ♳ (U+2673), ♴ (U+2674), ♵ (U+2675), ♶ (U+2676), ♷ (U+2677), ♸