0 and "B"0 which contain, respectively, all keys mapping to 0 and all keys mapping to 1. Then, to determine which value a given key maps to, we look it up in both filters. If it is in neither, then the key is not in the map. If the key is in "A"0 but not "B"0, then it does not map to 1, and has a high probability of mapping to 0. Conversely, if the key is in "B"0 but not "A"0, then it does not map to 0 and has a high probability of mapping to 1.