We define the Bott-Duffin decompositions of elements in a ring, which generalize the strongly clean decompositions, and prove that the Bott-Duffin decompositions of 1 − ab are in a natural bijection with those of 1 − ba. This bijection respects a number of well known additive decompositions of elements in a ring. For instance, the result implies that 1 − ab is strongly clean (respectively, strongly nil-clean, Drazin invertible, quasipolar, or pseudopolar) if and only if so is 1 − ba. Examples and further applications are given. C(S) := {r ∈ R : rs = sr for all s ∈ S}.We write C 2 (S) for the double-centralizer C(C(S)), and when S = {s} is a single element, we write C(s) rather than C({s}). Proposition 1.1. Fix an element α ∈ R = End(M k ). Given x ∈ R, there exists an additive decomposition α = e + x, where e ∈ idem(R) ∩ C(α), if and only if there exists a direct sum decomposition diagramProof. (⇒): Assume α = e + x for some idempotent e 2 = e ∈ R which commutes with α. Since x = α − e, we see that e commutes with x as well. We