  Let $K$ be an arbitrary infinite field. The cohomology group $H^2(SL(2,K),
H_2\,SL(2,K))$ contains the class of the universal central extension. When
studying representations of fundamental groups of surfaces in $SL(2,K)$ it is
useful to have classes stable under deformations (Fenchel--Nielsen twists) of
representations. We identify the maximal quotient of the universal class which
is stable under twists as the Witt class of Nekovar. The Milnor--Wood
inequality asserts that an $SL(2,{\bf R})$-bundle over a surface of genus $g$
admits a flat structure if and only if its Euler number is $\leq (g-1)$. We
establish an analog of this inequality, and a saturation result for the Witt
class. The result is sharp for the field of rationals, but not sharp in
general.
