  Let $X$ be a complex projective manifold. Fix two ample line bundles $H_0$
and $H_1$ on $X$. It is the aim of this note to study the variation of the
moduli spaces of Gieseker semistable sheaves for polarizations lying in the
cone spanned by $H_0$ and $H_1$. We attempt a new definition of walls which
naturally describes the behaviour of Gieseker semistability. By means of an
example, we establish the possibility of non-rational walls which is a
substantially new phenomenon compared to the surface case. Using the approach
of Ellingsrud and Goettsche via parabolic sheaves, we were able to show that
the moduli spaces undergo a sequence of GIT flips while passing a rational
wall.
