Six Degrees of Freedom - In engineering terms, refers to the ability to move three-dimensionally in space about the Cartesian coordinate system of X,Y and Z, including rotation about each axis. That is to say, an object can translate in all 3 planes, XY, YZ and XZ including rotation about the X,Y and Z axes. 6D's mission is to provide the public with superior safety helmets from both a technological perspective and a design perspective. They continually challenge the company to improve its technology and lead the marketplace with innovative designs. Replacement liner for the 6D Helmets ATR-1 Helmet. 6D's proprietary design provides optimum support, comfort and fit. Features genuine moisture-wicking fabric for a superior sweat management solution. Snaps in-and-out in seconds for fast and easy attachment and removal. Completely machine washable.