At current prices, the value of an aggregate is equal to the sum of its components. At constant prices, additivity requires this identity to be preserved for the extrapolated values of the aggregate and its components, when their values in some reference period are extrapolated to some other period using a set of interdependent volume index numbers, or, alternatively, when the values of an aggregate and its components in some period are deflated using a set of interdependent price index numbers based on some other period.