We characterize extensions of commutative rings R ⊂ S such that R ⊂ T is minimal for each R-subalgebra T of S with T = R, S. This property is equivalent to R ⊂ S has length 2. Such extensions are either pointwise minimal or simple. We are able to compute the number of subextensions of R ⊂ 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.2010 Mathematics Subject Classification. Primary:13B02,13B21, 13B22, 12F10; Secondary: 13B30.Key words and phrases. FIP, FCP extension, minimal extension, length of an extension, integral extension, support of a module, t-closure, algebraic field extension, separable field extension, principal subfield, pointwise minimal extension.