Position is one of the most useful and most complex of all the symbols in GD&T. The two methods of using Position discussed on this page will be RFS or Regardless of Feature Size and under a material condition (Maximum Material Condition or Least Material Condition). Position is always used with a feature of size.
A datum is theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well.
GD&T Flatness is a common symbol that references how flat a surface is regardless of any other datum’s or features. It comes in useful if a feature is to be defined on a drawing that needs to be uniformly flat without tightening any other dimensions on the drawing. The flatness tolerance references two parallel planes (parallel to the surface that it is called out on) that define a zone where the entire reference surface must lie.
The standard form of straightness is a 2-Dimensional tolerance that is used to ensure that a part is uniform across a surface or feature. Straightness can apply to either a flat feature such as the surface of a block, or it can apply to the surface of a cylinder along the axial direction. It is defined as the variance of the surface within a specified line on that surface.
GD&T Rule #1, also known as the Envelope principle, states that the form of a regular feature of size is controlled by its “limits of size." Limits of size, or otherwise known as size tolerances, can be seen in many forms. A few of them are symmetric, unilateral, and bilateral.
A countersink consists of a conical hole that is coaxial to a cylindrical hole, where the angle of the cone is determined by the fastener to be used. The purpose of a countersink is to allow a fastener, typically a flathead screw, to sit slightly below the surface of the part.