GD&T (geometric dimensioning and tolerancing)
also: GD&T · ASME Y14.5 · ISO GPS · datums
A standardized symbolic language on engineering drawings that defines allowed variation in form, orientation and location relative to datums, not just size.
Plus-or-minus dimensions alone are ambiguous: measured from where, and how flat or round? GD&T adds controls such as flatness, perpendicularity, position and profile, referenced to datums: the features that locate the part when it is assembled or inspected. In the US the governing standard is ASME Y14.5; internationally the ISO GPS family (including ISO 1101) covers similar ground with some differences in rules and symbols.
Done well, GD&T states exactly what matters for function and lets the supplier use any method that meets it, often permitting looser tolerances where function allows. It also gives inspectors an unambiguous definition of a good part.
Where it bites: over-applied GD&T inflates inspection cost, mixing ASME and ISO conventions confuses suppliers, and datums that don't reflect how the part actually mounts produce parts that are good on paper but don't fit.
Founder takeaway
Choose datums that match how each part locates in the assembly, and apply tight controls only where the stack-up shows they are needed.
engineer review pending