Concentricity

In GD&T, concentricity is a complex tolerance used to establish a tolerance zone for the median points of a cylindrical or spherical part feature.  Concentricity is generally reserved for high-precision parts, and only when there is a need to control median points.  Verifying concentricity is a time-consuming process, and in many cases it is recommended to use runout or position tolerance in lieu of concentricity. 

An example of concentricity is shown below.  In the top figure, the concentricity symbol is applied to the axis of the part.  The boxed symbols can be read "realtive to datum A, all median points of opposing elements on this cylindrical surface must lie within a cylindrical tolerance zone of 0.5".  The lower figure shows a part that meets this requirement.  The side view of the part shows the median points for each surface element and the end view shows the cylindrical tolerance zone.  The end view also shows a possible surface that meets the size limits of the part.