GD&T is a language for describing allowable geometric variation while protecting the function of a part. It complements size dimensions rather than replacing them.
Why ± dimensions are not enough
A rectangular coordinate scheme can indirectly control location but often creates ambiguous tolerance zones and poor functional interpretation. GD&T lets the drawing state what matters: flatness, perpendicularity, position, profile and other geometric relationships.
Datums create the reference frame
A datum reference frame represents how the part is oriented for inspection and, ideally, how it is constrained in the real assembly. Primary, secondary and tertiary datums progressively remove degrees of freedom.
Feature control frames
A feature control frame identifies the geometric characteristic, tolerance value, modifiers where applicable and datum references. Read the frame as a complete requirement rather than interpreting the symbol in isolation.
Size and geometry interact
A hole can satisfy its diameter tolerance but still be too far from its intended position. Conversely, a feature can be geometrically well located but wrong in size. Drawings frequently need both controls.
Do not decorate drawings with GD&T
Every control should protect a functional requirement, manufacturing need or inspection requirement. Adding tight controls everywhere makes a drawing expensive without necessarily improving the product.
Learning path
Start with datums, position, flatness, perpendicularity and profile. Once those are comfortable, learn material condition modifiers, datum modifiers and composite controls using real assembly examples.
Drawing & GPS verification
Confirm the drawing standard named on the product documentation and use the current applicable ISO GPS or ASME Y14.5 requirements.