Editing Cabibbo–Kobayashi–Maskawa matrix (section)

==Weak universality==
The constraints of unitarity of the CKM-matrix on the diagonal terms can be written as

::<math>\sum_k |V_{jk}|^2 = \sum_k |V_{kj}|^2 = 1</math>

separately for each generation {{mvar|j}}. This implies that the sum of all couplings of any ''one'' of the up-type quarks to ''all'' the down-type quarks is the same for all generations. This relation is called ''weak universality'' and was first pointed out by [[Nicola Cabibbo]] in 1967. Theoretically it is a consequence of the fact that all [[SU(2)]] doublets couple with the same strength to the [[vector boson]]s of weak interactions. It has been subjected to continuing experimental tests.