  Regarding the generating structure of the homogeneous ideal of a projective
variety $X \subset \mathbb{P}^r$, we define the rank index of $X$ to be the
smallest integer $k$ such that $I(X)$ can be generated by quadratic polynomials
of rank at most $k$. Recently it is shown that every Veronese embedding has
rank index $3$ if the base field has characteristic $\ne 2, 3$. In this paper,
we introduce some basic ways of how to calculate the rank index and find its
values when $X$ is some other classical projective varieties such as rational
normal scrolls, del Pezzo varieties, Segre varieties and the Pl\"{u}cker
embedding of the Grassmannian of lines.
