legally require permanently installed backflow preventers. Porous or perforated soaker hoses Special hoses designed to leak throughout their length are sometimes used to gently distribute water on a lawn or garden. These hoses have either many small holes drilled or punched in them, or are made of a porous material, such as sintered rubber particles. These "soaker hoses" are a simple, low-cost, substitute for a drip irrigation system. Expandable hoses These differ from traditional hoses in that the inner membrane expands when filled with water, much like a balloon. An outer cover protects the delicate expandable membrane from punctures. Such hoses "grow" when pressurized, and shrink back down when the pressure is released, allowing for easier storage. Standards and connectors Garden hoses connect using a male/female thread connection. The technical term for this arrangement is a "hose union". Spigots or sillcocks have male hose connectors only, and the mating end of a hose has a captive nut which fits the threads there. The thread standard for garden hose connectors in the United States, its territories, and Canada is known colloquially as "garden hose thread" (GHT), but its official designation is NH ("National Hose"): -11.5NH designates full form threads as produced by cutting material such as the brass spigot outlet or hose male or female end fitting found on more expensive hoses. -11.5NHR designates thin-walled couplers produced by rolling thin material, usually brass, typically found on less expensive hoses. -14NPSH designates female hose ends that mate a hose to a tapered pipe thread without a spigot. The US standard was defined by NFPA 1963, "Standard for Fire Hose Connections", then later by ANSI-ASME B1.20.7, which specifies diameter straight (non-tapered) thread with a pitch of 11.5 threads per inch (TPI). The female thread is abbreviated FHT (for "female hose thread"), and the male part is abbreviated MHT (for "male hose thread"). This fitting is used with -inch, -inch, and -inch hoses. In other countries, a British Standard Pipe (BSP) thread is used, which is and 14 TPI (male part outside diameter is ). The GHT and BSP standards are not compatible, and attempting to connect a GHT hose to a BSP fitting, or vice versa, will damage the threads. Various adaptors made of metal or plastic are available to interconnect GHT, BSP, NPT, hose barb, and quick connect fittings. Quick connectors In the 1980s, the use of quick-connector systems became increasingly popular.