  The central topic is this question: is a given $k$-\'etale algebra
$\prod_lE_l/k$ the specialization of a given $k$-cover $f:X\rightarrow B$ at
some point $t_0\in B(k)$? Our main tool is a {\it twisting lemma} that reduces
the problem to finding $k$-rational points on a certain $k$-variety. Previous
forms of this twisting lemma are generalized and unified. New applications are
given: a Grunwald form of Hilbert's irreducibility theorem over number fields,
a non-Galois variant of the Tchebotarev theorem for function fields over finite
fields, some general specialization properties of covers over PAC or ample
fields.
