Xblocs are typically applied on an armour slope steepness between 3V:4H and 2V:3H. Unlike natural rock, the hydraulic stability does not increase at shallower slope inclinations, because in that situation the interlocking effect is reduced. Standard Xbloc sizes vary between 0.75m3 (significant wave height up to Hs = 3.35m) and 20m3 (Hs = 10.0m). It shall be noted that the given relation between design wave height and volume size is valid for concept stage only. Further parameters as foreshore slope, crest configuration, construction equipment, etc. can have an important effect on the recommended unit size. For detailed design, in particular for non standard situations, physical model tests are essential and normally carried out to confirm overall stability and functional performance of a breakwater (wave overtopping and/ or wave penetration).