The present work establishes necessary and sufficient conditions for a nonlinear system with two inputs to be described by a specific triangular form. Except for some regularity conditions, such triangular form is flat. This may lead to the discovery of new flat systems. The proof relies on well-known results for driftless systems with two controls (the chained form) and on geometric tools from exterior differential systems. The paper also illustrates the application of its results on an academic example and on a reduced order model of an induction motor. where x ∈ U ⊂ R n is the state, U is open, n ≥ 2, u = (u 1 , u 2 ) ∈ R 2 is the control and f, g 1 , g 2 : U → R n are smooth (i.e. infinitely differentiable) mappings. Recall that (1) is driftless when f = 0 and that u = α(x)+β(x)v is a regular static state feedback defined on an open set V ⊂ U if the mappings α = (α 1 , α 2 ): V → R 2 and β = (β ij ): V → R 2×2 are smooth and the matrixIt was established in [1] necessary and sufficient conditions for (1) with f = 0 to be (locally) described around a given x 0 ∈ U by the chained formż