The special automorphism group of R[t]/(t m )[x 1 , . . . , x n ] and coordinates of a subring ofLet R be a ring. The Special Automorphism Group SAut R R[x 1 , . . . , x n ] is the set of all automorphisms with determinant of the Jacobian equal to 1. It is shown that the canonical map of SAutand Q ⊂ R is surjective. This result is used to study a particular case of the following question: if A is a subring of a ring B and f ∈ A [n] is a coordinate over B does it imply that f is a coordinate over A? It is shown that if A = R[t m , t m+1 , . . .] ⊂ R[t] = B then the answer to this question is "yes".Also, a question on the Vénéreau polynomial is settled, which indicates another "coordinate-like property" of this polynomial.