This paper examines the sketches for Henri Dutilleux's Symphony No. 2, Le Double, held at the Paul Sacher Stiftung in Basel. The material is described in more detail than has been attempted hitherto, testing and challenging some previous statements and conclusions. It also reveals sketches that have remained hidden, leading to some surprising new facts, including the extent of Dutilleux's indecision regarding Le Double's instrumentation and his struggle to compose part of the second movement, as well as an early sketch for a different opening to the work. It seeks, finally, to show how some of the newly discovered material has implications for how we view the finished composition.