A hanger bolt is used when it is necessary to fasten a metal part to a wooden surface. Hangar bolts (that resemble the threaded portion of a machine screw attached in-line to the threaded portion of a wood screw or lag bolt with no heads in evidence) are most commonly used in the light commercial and residential construction industry. It's wood threaded on one side and metal (nut) threaded on the other side such that you can drive it into the wood, then bolt something to the wood at varying heights.