Steel pipe (or black iron pipe) was once the most popular choice for supply of water and flammable gases. Steel pipe is still used in many homes and businesses to convey natural gas or propane fuel, and is a popular choice in fire sprinkler systems due to its high heat resistance.  In commercial buildings, steel pipe is used to convey heating or cooling water to heat exchangers, air handlers, variable air volume (VAV) devices, or other HVAC equipment.
Steel pipe is sometimes joined using threaded connections, where tapered threads (see National Pipe Thread) are cut into the end of the tubing segment, sealant is applied in the form of thread sealing compound or thread seal tape (also known as  PTFE or Teflon tape), and it is then threaded into a corresponding threaded fitting using two pipe wrenches.  Beyond domestic or light commercial settings, steel pipe is often joined by welding, or by use of mechanical couplings made by companies such as Victaulic or Anvil International (formerly Grinnell) that hold the pipe joint together via a groove pressed or cut (a rarely used older practice), into the ends of the pipes.
Other variations of steel pipe include various stainless steel and chrome alloys.  In high-pressure situations these are usually joined by TIG welding.
In Canada, with respect to natural gas (NG) and propane (LP gas), black iron pipe (BIP) is commonly used to connect an appliance to the supply.  It must however be marked (either painted yellow or yellow banding attached at certain intervals) and certain restrictions apply to which nominal pipe size (NPS) can be put through walls and buildings. With propane in particular, BIP can be run from an exterior tank (or cylinder) provided it is well protected from the weather, and an anode-type of protection from corrosion is in place when the pipe is to be installed underground.