There are many different types of collagen. Bone contains the collagen know as type I, which is also found in skin. The structure of most collagens is a triply-stranded helix. Each strand is a string of amino acids called an a chain. While every third amino acid is glycine, the other two can vary. With around twenty different amino acids occurring this could give rise to many different a chains, but only 25 have ever been identified. Even with this few it would still be possible to get thousands of different triple helixes, but collagen is thought to exist in only 15 or so of these combinations.