U-bolt

A U-bolt is a bolt in the shape of the letter U with screw threads on both ends.

For example, a 40 Nominal Bore U-bolt would be asked for by pipe work engineers, and only they would know what that meant.

In reality, the 40 nominal bore part bears little resemblance to the size and dimensions of the U-bolt.

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.

[2] Research has been done into the use of novel U-bolts to improve the interface shear resistance of steel-concrete composite beams.