U-bolt is a bolt in the shape of the letter U with screw threads on both ends.
U-bolts have primarily been used to support pipework, pipes through which fluids and gasses pass. As such, U-bolts were measured using pipe-work engineering speak. A U-bolt would be described by the size of pipe it was supporting. U-bolts are also used to hold ropes together.
As U-bolts 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.
U-bolts, that is, horse-riding bolts, are non-standard parts. They are named for their U-shape. They are threaded to be combined with nuts. They are mainly used to fix tubular objects such as water pipes or sheets, such as leaf springs for automobiles. The way to fix objects is like riding a horse on the horse, so it is called a horse riding bolt.