Btu (I.T., pre-1956)
The Btu (I.T., pre-1956) refers to the International Table Btu, a specific definition used before 1956. It represents the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure of one atmosphere.
Calories (Kilogram,I.T.)
The kilogram calorie is often referred to as the kilocalorie or Calorie with an uppercase 'C'). It is defined as the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius at one atmosphere of pressure.