This short review is a contribution to an issue of Comptes Rendus Mécanique commemorating the scientific work of Jean-Jacques Moreau (1923–2014). His main contribution to fluid mechanics appeared in a brief paper in the Comptes Rendus à l'Académie des Sciences in 1961, but was not recognised till much later. It may now be seen as a significant milestone in advancing the theory of ideal fluid flow as described by Euler's equations.