Dislocations are the crystalline defects responsible for the mechanical properties of conventional metals and alloys. When they become scarce or constrained, such as in nanocrystals1, grain-boundary-based mechanisms may compensate and lead to permanent deformation2,3. Shear-migration coupling is thought to be the most efficient of these processes4,5, but despite intense research activity, no consensus has emerged to quantify the possible shear produced by a migrating grain boundary6. Here we show experimentally that, in small-grained polycrystals, this shear does not depend on the grain boundary misorientation and that its efficiency remains low. These findings support a new concept of grain boundaries that may not be considered as crystalline defects carrying an intrinsic ‘coupling factor’7 (similarly to the Burgers vector of a dislocation) but rather as specific lattices containing peculiar defects, known as disconnections, that will, in turn, govern the properties, at least mechanical, of grain boundaries. They also confirm that polycrystals can plastically deform without dislocations but less effectively, providing a potential path to explain the poor ductility of nanocrystalline metals at low and room temperature.