The most common mechanism for suppression of the rate of gamma decay of excited atomic nuclei, and thus make possible the existence of a metastable isomer for the nucleus, is lack of a decay route for the excited state that will change nuclear angular momentum (along any given direction) by the most common (allowed) amount of 1 quantum unit ℏ ℏ of spin angular momentum. Such a change is necessary to emit a gamma-ray photon, which has a spin of 1 unit in this system. Integral changes of 2, 3, 4, and more units in angular momentum are possible (the emitted photons carry off the additional angular momentum), but changes of more than 1 unit are known as forbidden transitions. Each degree of forbiddenness (additional unit of spin change larger than 1, that the emitted gamma ray must carry) inhibits decay rate by about 5 orders of magnitude. The highest known spin change of 8 units occurs in the decay of Ta-180m, which suppresses its decay by a factor of 10 from that associated with 1 unit, so that instead of a natural gamma decay half life of 10 seconds, it has a half life of more than 10 seconds, or at least 3 x 10 years, and thus has yet to be observed to decay.