  We characterize extensions of commutative rings $R\subset S$ such that
$R\subset T$ is minimal for each $R$-subalgebra $T$ of $S$ with $T\neq R,S$.
This property is equivalent to $R\subset S$ has length 2. Such extensions are
either pointwise minimal or simple. We are able to compute the number of
subextensions of $R\subset S$. Besides commutative algebra considerations, our
main result is a consequence of the recently introduced by van Hoeij et al.
concept of principal subfields of a finite separable field extension. As a
corollary of this paper, we get that simple extensions of length 2 have FIP.
