$G$ acts, linearized by a very ample line bundle $\mathcal L$ giving a $G$-equivariant embedding $i:M \hookrightarrow {\mathbb{P}}(V)$. Fix a inner product on $X_*(T \otimes K_s)$ to be one which is invariant under the action of the Weyl group as well as the Galois group Gal$(K_s \mid K)$ (See \[Kempf\]).\
A point $m \in M$ is said to be semistable if it semistable after base change to its algebraic closure, i.e thought of as an element in $V (\bar{K})$.

Let $m \in M$ be a $K$-rational point of $M$. Let $P(m)$ be the instability parabolic of $m$ defined over $\bar{K}$. By invariance of the inner product under the Galois action and uniqueness of $P(m)$ we see that if $P(m)$ is defined over $K_s$, then it is already defined over $K$. \[See RR\].\
[**Rationality of the instability parabolic and its consequences**]{}\
Let $X$, $G$ and $\mathcal L$ be as before. Suppoe $\rho: G \rightarrow Gl(V)$ be a representation of $G$ which takes the connected component of the centre of $G$ to the centre of $Gl(V)$. Let $P$ be a maximal parabolic of $Gl(V)$. Choose the very ample generator $\mathcal L$ of $Gl(V)/P$. This is a linearized very ample line bundle giving 