Abstract: A connector for joining at least three tubes and at least two panels in a rectangular structure, comprising a body having at least three and not more than six pins, each for engaging the inside of a tube and extending in a direction either perpendicular to or opposite to every other pin, a recess between each pair of mutually-perpendicular pins for receiving a corner of a panel, each recess being defined by opposed parallel surfaces of a pair of spaced panel-engaging members, each of which members also has two tube-engaging surfaces each for abutting with the outside of a tube fitted to one of the pins of the said pair of mutually-perpendicular pins.