A penthouse is an apartment or unit on the highest floor of an apartment building, condominium, hotel or tower. Penthouses are typically differentiated from other apartments by luxury features. The term 'penthouse' originally referred, and sometimes still does refer, to a separate smaller 'house' that was constructed on the roof of an apartment building. Architecturally it refers specifically to a structure on the roof of a building that is set back from its outer walls. These structures do not have to occupy the entire roof deck. Recently, luxury high rise apartment buildings have begun to designate multiple units on the entire top residential floor or multiple higher residential floors including the top floor as penthouse apartments, and outfit them to include ultra-luxury fixtures, finishes, and designs which are different from all other residential floors of the building. These penthouse apartments are not typically set back from the building’s outer walls, but are instead flush with the rest of the building and simply differ in size, luxury, and consequently price. High-rise buildings can also have structures known as mechanical penthouses that enclose machinery or equipment such as the drum mechanisms for an elevator.