  We give an overview of the general framework of forms of Bak, Tits and Wall,
when restricting to vector spaces over fields, and describe its relationship to
the classical notions of Hermitian, alternating and quadratic forms. We then
prove a version of Witt's lemma in this context, showing in particular that the
action of the group of isometries of a space equipped with a form is transitive
on isometric subspaces.
