  The effects of a magnetic field on a modulated phase is studied. A modulated
phase is found to have two critical fields $H_{1}$ and $H_{2}$. For a large
enough magnetic field. $H_1$ can be approximated by a linear law. As a result,
the minimum magnetic field needed to destroy a modulated phase is a constant.
The minimum magnetic field also greatly depends on the order of a commensurate.
A very high order commensurate phase and an incommensurate phase are impossible
to survive a magnetic field. The behaviour of a magnetoelastic chain in a
magnetic field is can be described by a harmless Devil's staircase. The inverse
temperature is found to play a role similar to that of a special magnetic
field. The deeper physics underlying these new phenomena is the breaking of the
left-right symmetry of a phase diagram. As a result a controllable path to a
modulated phase is found.
