Which tool is a heavy weight that indicates vertical trueness?

A plumb bob is a tool designed to indicate vertical trueness, which means it helps determine whether a surface is perfectly vertical or not. The plumb bob consists of a pointed weight suspended from a string; when the weight hangs freely, it will align itself with the force of gravity, providing a true vertical reference. This is essential in construction and carpentry to ensure that walls, posts, and other structures are correctly aligned vertically.

The other tools listed serve different purposes: the penny system is related to leveling and measuring, a square is used for checking right angles, and a sliding T-bevel is designed to duplicate angles. While all these tools are valuable in their respective applications, they do not serve the function of indicating vertical alignment like a plumb bob does.