text: When writing sequences for systematic biology it may be necessary to have IUPAC codes that mean "any of the two" or "any of the three". The IUPAC code R (any purine) is complementary to Y (any pyrimidine) and M (amino) to K (keto). W (weak) and S (strong) are usually not swapped but have been swapped in the past by some tools. W and S denote "weak" and "strong", respectively, and indicate a number of the hydrogen bonds that a nucleotide uses to pair with its complementing partner. A partner uses the same number of the bonds to make a complementing pair.
  An IUPAC code that specifically excludes one of the three nucleotides can be complementary to an IUPAC code that excludes the complementary nucleotide. For instance, V (A, C or G - "not T") can be complementary to B (C, G or T - "not A").