  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.
