is the “gold standard” for domestic water systems. Traditionally used, expensive, fussy, all metal fittings usually sweated and meets all codes. If properly installed may also serve as a central household electrical ground under some codes. Copper may be rigid or soft, comes in several grades (M,L,K and more), and fittings may be copper or brass. There are isolation fittings (break the electrical continuity in a run of pipe) for special applications. Copper pipe has the highest pressure and temperature ratings of all normal-use domestic water pipe options. Copper is also prone to freezing and does not like to be bent once installed. Copper is also UV, cold, and heat resistant, and may be direct- buried if the proper grade.