What is the purpose of a 123 block and how is it used?

they can be used for all sorts of things, i use them for setup blocks, its nice because they are exactly 1 inch by 2 inch by 3 inches (unless they have been messed with) you can use them to block parts up a certain amount to machine surfaces, the ones with tapped holes come in handy to, you can clamp them in a vise and use the screw holes to bolt parts to for surface grinding and machining, drilling and tapping
its endless the uses in a machine shop

thats the best i can explain it

Ron Smith

Good explanation Ron. They are ground square and parallel. so along with the dimensions given by Ron they can be used in a multitude of configerations in an accurate fashion.

Hope that helps, Jay