Sometimes called Banjo, or Anchor style connectors, Universal Fasteners are the traditional way to connect aluminum extrusion profiles.  The base piece of extrusion needs a properly sized hole to receive the body of the universal fastener.  Typically, when building with these connectors, it's necessary to loosly pre-load the head of the fastener into the adjacent extrusion's slot first, and then attaching the body of the universal fastener into the machined counterbore on the base piece of extrusion.  Tightening the set screw will create a strong connection that prevents rotation.