text: When , the decay is referred to as "forbidden". Nuclear selection rules require high  values to be accompanied by changes in nuclear spin () and parity (). The selection rules for the th forbidden transitions are:
  where  corresponds to no parity change or parity change, respectively. The special case of a transition between isobaric analogue states, where the structure of the final state is very similar to the structure of the initial state, is referred to as "superallowed" for beta decay, and proceeds very quickly. The following table lists the Δ and Δ values for the first few values of :