Let
be a field of
characteristic different from
,
and let
be quaternion
algebras over
such
that any element in
generated by
has index
at most
. For the triple
we construct a certain
invariant, lying in
, which
is a
-fold Pfister form,
provided the algebras
have a common slot. Among other results we prove that the algebras
have a common slot if and only if the torsion of the group
is zero,
where
is the projective conic associated with the algebra
.