Straightness of a median line defines a tolerance zone in which the derived median line must lie. When specified at MMC, it defines a virtual condition boundary which the surface must not violate. Note the feature control frame is placed below the size tolerance to make it a median line control. It is most common to use straightness of a median line with an MMC modifier.

Straightness – line elements may be applied to the surface of a pin. The specification defines a tolerance zone in which the line elements on the surface must lie. The feature control frame is directed to the surface. MMC and LMC modifiers are not allowed.

Cylindricity is the condition of a surface of revolution within which all points of the surface are equidistant from a common axis.

Flatness is the condition of a surface having all elements in one plane. A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface must lie.

Circularity is a 2D tolerance. It controls circular elements, not longitudinal elements. Each circular element of the surface in a plane perpendicular to the axis must lie between two concentric circles, one having a radius 0.03 larger than the other. In addition, the feature must be within the limits of size, including perfect from at MMC.