Hydrogen symmetry problem. Symmetries resulting from interchangeable hydrogens. The figure presents three successive atom assignments within an ITS mapping. Bonds present in I are given in black, bonds to be formed to derive O are dotted and gray. The ITS describes the loss of a hydrogen for the carbon (bond order decrease) and the bond formation between the decoupled hydrogen with the oxygen next in the ITS. It becomes clear that all 4 hydrogens are not distinguishable, which results in 4 possible symmetric ITS mappings.