 Although knitting needle diameter is often measured in millimeters, there are several different size systems, particularly those specific to the United States, the United Kingdom and Japan; a conversion table is given in the knitting needle article.Such knitting needles may be made out of any materials, but the most common materials are metals, wood, bamboo, and plastic.Different materials have different frictions and grip the yarn differently; slick needles such as metallic needles are useful for swift knitting, whereas rougher needles such as bamboo are less prone to dropping stitches.