The neck blank is made of curly maple (although normally, Basswood is used).  The board is cut into three pieces so that the middle piece is 1/8 inch thick.  Two contrasting pieces of hardwood are inserted to add stiffness to the neck.  Usually, woods such as ebony, rosewood or wenge are used. 

Titebond or epoxy can be used to glue the pieces together.  Assemble the neck blank, clamp the pieces together and set aside to dry.