  Divergence-free symmetric tensors seem ubiquitous in Mathematical Physics. We
show that this structure occurs in models that are described by the so-called
"second" variational principle, where the argument of the Lagrangian is a
closed differential form. Divergence-free tensors are nothing but the second
form of the Euler--Lagrange equations. The symmetry is associated with the
invariance of the Lagrangian density upon the action of some orthogonal group.
