There are fourteen geometric characteristic symbols in common use, and each one answers a different question about how far a feature may stray. Reading a feature control frame is a left to right operation: the symbol says what is controlled, the number says how much, the material condition modifier says whether the allowance grows, and the letters say what it is all measured from.
Form: four symbols, no datum
Straightness, flatness, circularity and cylindricity control a feature against itself, so none of them takes a datum. Straightness bounds a line element, flatness bounds a whole surface between two parallel planes, circularity bounds one cross section of a round feature, and cylindricity bounds the whole length at once. Cylindricity is the expensive one because it controls roundness, straightness and taper together.
Orientation: three symbols, always with a datum
Perpendicularity, angularity and parallelism all say how a feature sits relative to something else, so all three require at least one datum reference. The tolerance zone is usually two parallel planes, or a cylinder when the callout applies to an axis. Getting the datum wrong here is the most common drawing error, because the number looks reasonable and the reference does not match the fixture.
Location and profile: where the money is
Position places a feature relative to a datum reference frame and is the workhorse of hole patterns. Profile of a line and profile of a surface bound a whole shape against its theoretical form and can carry both form and location in one callout, which is why moulded and cast parts lean on profile so heavily. Concentricity and symmetry survive on older prints and are being retired in favour of position and profile.
Runout, modifiers and the rest of the frame
Circular runout and total runout combine form and orientation for rotating parts measured against a datum axis, and they are cheap to check because an indicator and a pair of centres will do it. The modifiers matter as much as the symbols: maximum material condition grants bonus tolerance as the feature departs from its worst case size, and a projected tolerance zone moves the control into the mating part rather than the hole.
Questions people ask about gd&t symbols
How many GD&T symbols are there?
Fourteen geometric characteristics in the current US practice, grouped as form, orientation, location, profile and runout, plus a small set of modifiers and datum symbols that qualify them.
What is the difference between a symbol and a feature control frame?
The symbol is one box. The feature control frame is the whole strip: the characteristic symbol, the tolerance value with any modifier, and the datum references in order of precedence.
Which symbols should a first drawing use?
Position for hole patterns, profile of a surface for shaped faces, flatness for a mating face and perpendicularity for anything that must stand square. Those four cover the majority of commercial work.
Do the symbols change what a part costs?
Indirectly. Each frame is an inspection operation, and tight values narrow the process window. A drawing with four well chosen frames is cheaper to buy than one with twenty defensive ones.