The phosphorous atom in phosphorous acid (H 3 PO 3 ) illustrates the use of Algorithm 1 for atoms having multiple target valences. Bond order sum is four (2+1+1). The first default valence is 3, but subtracting valence from that value yields a negative number . Continuing to the next default valence, five, a difference of 1 is obtained. Therefore, the subvalence of the phosphorous-bearing atom is reported as one.