The term 'STAIR's Vertical Piece' is a reference to the risers of a staircase. Risers are the vertical pieces that connect the treads, which are the horizontal pieces, of a staircase. They are typically made of wood or metal and can be decorated with various designs. The term 'RISER' is used to describe the vertical piece of a staircase and is also used in other contexts such as architecture and engineering.