A concrete anchor is a piece of steel used to transfer loads to concrete. They can be cast into the concrete or put into a piece of hardened concrete later. There are several kinds of cast-in anchors, but headed bolts, hooked bolts (J- or L-bolts), and headed studs are the most common. The expansion anchor, the undercut anchor, and the sticky anchor are the three anchors used after the fact. In adhesive anchors, steel parts like threaded rods and reinforcing bars that have been bent or internally threaded steel sleeves with bends on the outside are used. Anchor systems often join two structural parts or attach a non-structural part to a building.