Numerous systems are used to secure containers aboard ships, depending on factors such as the type of ship, the type of container, and the location of the container.[36][37] Stowage inside the holds of fully cellular (FC) ships is simplest, typically using simple metal forms called container guides, locating cones, and anti-rack spacers to lock the containers together.[38] Above-decks, without the extra support of the cell guides, more complicated equipment is used.[36] Three types of systems are currently in wide use: lashing systems, locking systems, and buttress systems.[36] Lashing systems secure containers to the ship using devices made from wire rope, rigid rods, or chains and devices to tension the lashings, such as turnbuckles.[36] The effectiveness of lashings is increased by securing containers to each other, either by simple metal forms (such as stacking cones) or more complicated devices such as twist-lock stackers.[36] A typical twist-lock is inserted into the casting hole of one container and rotated to hold it in place, then another container is lowered on top of it.[39] The two containers are locked together by twisting the device's handle.[39] A typical twist-lock is constructed of forged steel and ductile iron and has a shear strength of 48 tonnes.[40]