Unlike compact globular proteins, fibrous proteins are often located in the extracellular matrix, provide structure, and form extended shapes. Fibrous proteins are usually composed of either alpha-helices or beta-sheets but rarely a mix of both. They often have hydrophobic amino acids on their outer surfaces that interact with other monomers to form larger structures. For example, collagen is a fibrous protein composed of an extended alpha helix. Three helices wind around each other to make a structure called a coiled-coil. Collagen fibrils provide structure and flexibility in connective tissues. The protein fibroin is an example of a fibrous protein composed of beta-sheets and is the protein in silk which allows the silk strands to be both flexible and strong.