A counting rule for the size of a union: |A ∪ B| = |A| + |B| − |A ∩ B|, subtracting the overlap that would otherwise be double-counted. For three sets it adds the triple overlap back: |A ∪ B ∪ C| = |A| + |B| + |C| − |A∩B| − |A∩C| − |B∩C| + |A∩B∩C|.