The paper deal with the noncommutative Fréchet * -algebra L(s ′ , s) of the so-called smooth opertors, i.e. linear and continuous operators acting from the space s ′ of slowly increasing sequences to the Fréchet space s of rapidly decreasing sequences. By a canonical identification, this algebra of smooth operators can be also seen as the algebra of the rapidly decreasing matrices. We give a full description of closed commutative * -subalgebras of this algebra and we show that every closed subspace of s with basis is isomorphic (as a Fréchet space) to some closed commutative * -subalgebra of L(s ′ , s). As a consequence, we give some equivalent formulation of the long-standing Quasi-equivalence Conjecture for closed subspaces of s.