u, d

, so its strong breaking is controlled by the small difference

m_{d} - m_{u}

. U-spin exchanges

d, s

, so its strong breaking involves the much larger

m_{s} - m_{d}

. Relative to the characteristic strong-interaction scale, the latter is much larger; this explains why U-spin is a substantially poorer symmetry.

Flavour-dependent interactions are a second source. Electromagnetism distinguishes

u

from

d

because

Q_{u} = 2/3

and

Q_{d} = - 1/3

, and therefore breaks isospin. However,

d, s

both have charge

- 1/3

, so their electromagnetic couplings alone do not break U-spin in the equal-mass limit. If two breaking mechanisms for each symmetry are required, the second common mechanism is the weak interaction: the CKM matrix and weak flavour transitions distinguish the flavours and do not conserve either strong-interaction flavour symmetry. The dominant difference in the quality of the two approximate strong symmetries is still the mass splitting.