U Clamp

A U Clamp is a bolt in the shape of the letter U with screw threads on both ends. U Clamps have primarily been used to support pipework, pipes which fluids and gasses pass through. As such, U Clamps were measured using pipe work engineering speak. A U Clamp would be described by the size of pipe it was supporting. As U Clamps are now being used by a much wider audience to clamp any kind of tubing / round bar then a more convenient measurement system needs to be used.
Four elements uniquely define any U Clamp:
Material Type (For example: Bright zinc plated mild steel)
Thread dimensions (For example: M12 * 50 mm)
Inside diameter (For example: 50 mm - the distance between the legs)
Inside height (For example: 120 mm)