A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances. They are usually synthetic, most commonly derived from petrochemicals, but many are partially natural. Most plastics contain organic polymers. The vast majority of these polymers are based on chains of carbon atoms alone or with oxygen, sulfur, or nitrogen as well. The backbone is that part of the chain on the main 'path' linking a large number of repeat units together. To customize the properties of a plastic, different molecular groups 'hang' from the backbone (usually they are 'hung' as part of the monomers before the monomers are linked together to form the polymer chain). The structure of these 'side chains' influence the properties of the polymer.