displayed with the arrows circulating clockwise, but the underlying Möbius strip exists in two topologically distinct mirror-image forms of opposite handedness. The American Paper Institute originally promoted four different variants of the recycling symbol for different purposes. The plain Möbius loop, either white with an outline or solid black, was to be used to indicate that a product was recyclable. The other two variants had the Möbius loop inside a circle—either white on black or black on white—and were meant for products made of recycled materials, with the white-on-black version to be used for 100% recycled fiber, and the black-on-white version for products containing both recycled and unrecycled fiber. For example, a paper envelope might have both the first and last of these four symbols, to indicate that it was recyclable, and made from both recycled and unrecycled fibers. In addition to the resin identification codes 1–7 in the triangular recycling symbol, Unicode lists the following recycling symbols: (indicates product contains recycled paper) (indicates product contains partially recycled paper) (e.g. for acid-free paper) An ISO/IEC working group has researched and documented some of the variations of the recycling logo in use during 2001, and has made recommendations for adding some more of them to the Unicode standard. With the rapid expansion of materials converted to printer filament for 3-D printing using recyclebot technology, a large expansion of resin identification codes has been proposed. Resin identification code In 1988, the American Society of the Plastics Industry (SPI) developed the resin identification code which is used to indicate the predominant plastic material used in the manufacture of the product or packaging. Their purpose is to 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), ♸ (U+2678), ♹ (U+2679), and ♺ (U+267A). Recycling codes extend these numbers above 7 to include various non-plastic materials, including metals, glass, paper, and batteries of various types.