Geometric Dimensioning and Tolerancing (GD&T) can look intimidating on a drawing, but almost everything on it traces back to one idea: datums. A datum is a reference — a theoretically exact point, axis, or plane — that other features on a part are measured against. Instead of dimensioning every feature relative to every other feature, GD&T lets you pick a small set of datums and control everything else relative to them, which is closer to how the part is actually held and measured in manufacturing and inspection. Datums are usually labeled with a letter in a small box on the drawing, and referenced inside feature control frames using that letter. A typical part will have a primary, secondary, and tertiary datum, forming what's often called a datum reference frame — together they lock down all six degrees of freedom. Picking good datums usually means thinking about how the part will actually be fixtured during machining and inspection, not just what's easiest to dimension on paper. Get the datum scheme right early, and the rest of the tolerancing tends to fall into place.