In quantum mechanics, angular momentum is quantized. The various multipole fields have particular values of angular momentum: E ℓ ℓ radiation carries an angular momentum ℓ ℓ in units of ℏ ℏ ; likewise, M ℓ ℓ radiation carries an angular momentum ℓ ℓ in units of ℏ ℏ . The conservation of angular momentum leads to selection rules, i.e., rules defining which multipoles may or may not be emitted in particular transitions.