The fraction of any change in income not consumed is, by definition, the marginal propensity to save (MPS). This explains why the marginal propensity to consume (MPC) plus the MPS of any change equals one, as they reflect the total change in income being either consumed or saved:
\[ MPC + MPS = 1 \]
Thus, if individuals increase their income and choose to consume a certain fraction of that increase, the remainder that is not consumed is saved, represented by the MPS.