The smectic A is the simplest lamellar phase, as the layers are fluid and the molecules are perpendicular to the layers. This phase has the same point symmetry as the usual nematic phase (same rotations and reflections); it is therefore optically uniaxial, with optical axis parallel to the director n normal to the layers. On the other hand, its translation symmetry is lower than for a nematic. This symmetry reduction has very important consequences on the nature of the defects one can encounter in this phase (see chapter C.III) and on its rheological behavior. Indeed, the layers can slide one over the other, but will resist compression and dilation. A smectic phase is thus essentially viscoelastic, behaving as an usual viscous fluid in the plane of the layers and as an elastic solid under compression or dilation normal to the layers.