A hollow structural section (HSS) is a type of metal profile with a hollow tubular cross section. The term is used predominantly in USA, or other countries which follow US construction or engineering terminology. In the UK, or other countries which follow British construction or engineering terminology, the term HSS is not used. Rather, the three basic shapes are referenced as CHS, SHS, and RHS, being circular, square, and rectangular hollow sections. Typically, these designations will also relate to metric sizes, thus the dimensions and tolerances differ slightly from HSS.