Most lighting fixtures accept the standard top stud on light stands, which meaures 5/8 inches in diameter. For those fixtures that accept nothing larger than 3/8 in studs, order the Manfrotto 3301 Rapid Adapter (5/8" to Long 3/8" Diameter Stud) to fit your fixture onto your standard light stand.
Works great for hanging my canvas backgrounds.
I have 2 continuous tungsten lights that did not fit on on a standard 5/8 stud. This adapter not only allows me to use the lights on a standard light stand but it also gives me the option to mount it on a 90 degree angle... not that I've found any need to do so, but it's nice to be able to if that need ever arises.