So change in enthalpy, okay? So standard enthalpy of formation and it basically means it's the change in enthalpy for a reaction that creates 1 mol of compound from its constituent elements. For example, let's take the creation or formation of water from its constituents, from hydrogen and from oxygen, okay? These guys combine to form water and there is a decrease in enthalpy. And that means that the internal energy of the system decreases. There are less molecules within the system and so there is less Enthropy within the system.