Many quantities in Thermodynamics (energy, E; temperature, T; enthalpy, H; entropy, S; free energy, G) are usually measured as differences between their values before and after a process has occurred. These are written using the Greek symbol D (delta), which is read "change in". So DE is "change in energy", DT is change in temperature, and so on. How are these D's calculated?

Since all of these quantities are state functions, their D's across any process depend only on their inital and final values and not on how the system got from its initial state to its final state. So, for example,

Or the change in energy is the value of the energy at the end of the process minus the value of the energy at the start of the process. Similarly,

Or the change in temperature is the temperature at the end of the process minus the temperature at the start of the process.