Alternately spelled "cheeseboro" or "cheesborough". Colloquially, "Cheese-burger" or "burger." Also known as "scaffold clamp." A device for connecting iron pipe or aluminum tube, usually to create a lighting position. Comes in two styles: Rigid (or fixed 90°), and Swivel; and made from two materials: forged steel, or machined aluminum alloy. The steel variety almost always has 15/16" hex nuts, and the aluminum, wing-nuts. The steel variety comes in sizes to fit 1.25", 1.5", and 2" I.D. pipe. The aluminum fits 50mm tube. Thus for greatest strength, every effort should be made to use steel with steel pipe, and aluminum with aluminum tube.
The aluminum Swivel Coupler, from The Light Source.
A steel Rigid/Fixed Cheeseborough, from Clamps Unlimited. (Note: the website has its descriptions for Double/Right Angle Coupler and Swivel Coupler reversed.)
From another forum, the likely origin of the term:
Interesting that of all the alternate spellings, "Chesebrough" is the least common. Some claim that Robert Chesebrough, inventor of petroleum jelly (Vaseline), to have also invented the scaffold clamp.