The strain rate can also be expressed by a single number when the material is being subjected to parallel shear without change of volume; namely, when the deformation can be described as a set of infinitesimally thin parallel layers sliding against each other as if they were rigid sheets, in the same direction, without changing their spacing. This description fits the laminar flow of a fluid between two solid plates that slide parallel to each other (a Couette flow) or inside a circular pipe of constant cross-section (a Poiseuille flow). In those cases, the state of the material at some time t can be described by the displacement X ( y , t ) of each layer, since an arbitrary starting time, as a function of its distance y from the fixed wall. Then the strain in each layer can be expressed as the limit of the ratio between the current relative displacement X ( y + d , t ) − − X ( y , t ) of a nearby layer, divided by the spacing d between the layers: