  In this paper we present a proof for automatic O(a) improvement in twisted
mass lattice QCD at maximal twist, which uses only the symmetries of the
leading part in the Symanzik effective action. In the process of the proof we
clarify that the twist angle is dynamically determined by vacuum expectation
values in the Symanzik theory. For maximal twist according to this definition,
we show that scaling violations of all quantities which have non-zero values in
the continuum limit are even in a. In addition, using Wilson Chiral
Perturbation Theory (WChPT), we investigate this definition for maximal twist
and compare it to other definitions which were already employed in actual
simulations.
