V-Block

The “v” block is primarily used for securing pipes, rods, etc. to enable work to be carried out on them. They often come with screw clamps to hold the work. There are also versions with internal magnets for magnetic work-holding. V-blocks are usually sold in pairs. 

V-blocks can also be used to hold other shaped material for marking or machining

Here are some good illustrations

A V-block can be used in conjunction with a surface table to find the centre of parts. This can also be done with round bar