GD&T is geometric dimensioning and tolerancing: a symbolic language on an engineering drawing that says how far each feature may depart from perfect, and measures that departure from a stated reference rather than from another feature that is itself allowed to move. It is the difference between a drawing that can be inspected and one that two competent people read two different ways.
Why plus minus dimensioning runs out
A stack of plus and minus dimensions tells you the size of things and almost nothing about their relationship. Two holes can each sit inside tolerance and still refuse the mating part, because nothing on the drawing said what they are located from, or how round or how square they have to be. Geometric callouts fix a reference first and then bound the wander from it, so the inspection has an answer.
Datums are the whole trick
A datum is the surface or axis a measurement starts from, and the order you name them decides how the part sits in the fixture. A drawing whose datums do not match how the assembly actually locates the part will pass parts that do not fit and reject parts that do. Choosing datums is a design decision rather than a drafting one, and it belongs with whoever knows the assembly.
The controls you will meet first
Form controls bound a feature against itself: flatness, straightness, circularity, cylindricity. Orientation controls bound it against a datum: perpendicularity, angularity, parallelism. Location controls place it: position, concentricity, symmetry. Profile controls do both at once across a surface. Most commercial drawings live on position, profile, flatness and perpendicularity and never need the rest.
Where it saves you money
Used well it widens tolerances rather than narrowing them, because bonus tolerance at maximum material condition gives back what a rigid plus minus scheme threw away. Shops quote geometric callouts as inspection work, so a drawing that controls only what matters is cheaper to buy than one that sprays tight limits across every feature out of caution.
Questions people ask about gdt
What does GD&T stand for?
Geometric dimensioning and tolerancing. It is often typed without the ampersand as GDT, which means the same thing, and the governing US standard is ASME Y14.5.
Do I have to use it on every drawing?
No. A simple bracket with generous clearances does not need it. Use it where fit, function or interchangeability depends on a relationship between features, and leave everything else on ordinary limits.
Does it make parts more expensive?
The symbols do not. Tight values do, and so does inspection, because every geometric callout is something a shop must measure and record. Control what the assembly needs and nothing more.
Which standard should a US drawing cite?
ASME Y14.5 is the usual reference for drawings made and bought in the United States. Say on the title block which revision applies, because the interpretation of a few symbols has changed between editions.