The Wondrous 1-2-3 Block
...if a tool is square. To clamp pieces of wood together to guarantee squareness when gluing. To attach a machinist’s dial (a another inexpensive tool that will get its own post). For woodworking, a cheap set from Amazon will work fine. I assume machinists will want to seek out higher quality 1-2-3 blocks. They make a great, if heavy, stocking stuffer for the precision impaired members of your household . . ....