◎ Concentricidad

Controls the real location of a feature of size (hole, shaft, slot, median plane) relative to a datum system, comparing its real position with its exact theoretical position (basic dimension). The tolerance zone is normally a cylinder (with ⌀, for holes or shafts) or two parallel planes (for slots or median planes) centered exactly on the basic position. It advantageously replaces the classic ± coordinate tolerancing: it defines a real (circular) acceptance zone, not a square, giving more usable area for the same functional control.

Controls that a feature (face, axis or median plane) forms 90° to the reference datum. It's an orientation tolerance: it says nothing about the form of the feature itself (a face can be perfectly perpendicular on average and still not be flat, which is why it's wise to also state flatness if applicable). With ⌀ before the value, the tolerance zone is cylindrical around the theoretically perpendicular axis; without it, it's two parallel planes oriented at 90° to the datum.

Controls that a feature (face, axis or median plane) stays parallel to the datum, with no theoretical angle other than 0°. The tolerance zone is two planes (or a cylinder, if it carries ⌀) parallel to the datum, separated by the stated value. As with perpendicularity, it doesn't control the feature's own form: a face can be parallel on average to its datum and still not be flat.

Limits the variation of a real surface between two theoretical parallel planes separated by the tolerance value: the whole surface must fit between both planes, regardless of its orientation to other faces of the part (that's why it carries no datum). Passing flatness doesn't guarantee the face is square or parallel to another — perpendicularity and parallelism control that separately.

Combines in a single requirement the roundness of all cross-sections, the straightness of the generatrices (longitudinal lines) and the taper along the whole cylindrical surface. The tolerance zone is two coaxial cylinders radially separated by the stated value, within which the entire real surface must stay. Requiring no datum, it's the most complete cylindrical form tolerance and also the most expensive to achieve and inspect.

Limits the radial variation of a circular cross-section from a perfect circle, evaluated in a plane perpendicular to the axis at a specific point — not along the whole part (that's what cylindricity is for). The tolerance zone is two concentric circles radially separated by the stated value; the real section must stay between them. It needs no datum: it's roundness of that section relative to itself, not to an external reference axis.

Limits the deviation of a real line (a generatrix, an edge or the axis of a cylinder) from a theoretically perfect straight line. The tolerance zone is the space between two parallel straight lines separated by the stated value, evaluated point by point along that line. It's a pure form tolerance: by default it requires no datum, and it controls only the specific line indicated on the drawing (an edge, a generatrix seen in profile), not necessarily the whole axis of the part — that's what axis straightness is for.

Controls the orientation of a feature relative to a theoretical angle (basic dimension, boxed) other than 0° or 90° to a datum — for those two specific cases parallelism and perpendicularity are used. The tolerance zone is a band oriented exactly at the basic angle: the tolerance value is always expressed in linear units (mm), like all other GD&T tolerances, never in degrees or minutes.

Controls the coincidence of the derived axis of a cylindrical feature with the axis of an also-cylindrical datum. Unlike concentricity (which compares centers of individual sections), coaxiality compares the feature's complete axis with the datum axis, treating both as derived straight lines.

Controls that a median feature (slot, tab, median plane) is centered on the datum's median plane. The tolerance zone is two parallel planes, equidistant from the datum's median plane, within which the real median plane of the controlled feature must fall.

Two or more rows within the same geometric tolerance frame, sharing symbol and datums but with different values: the upper row (PLTZF, pattern-locating tolerance zone) controls the location of the complete pattern relative to the datum system; the lower row (FRTZF, feature-relating tolerance zone) controls only the geometric relationship between the pattern's features among themselves, usually with a tighter tolerance. They are not two independent requirements: the lower row's zone must be contained within the upper row's zone at each feature.

Indicates, via the numeric prefix (4×, 6×, etc.) or the "PATTERN" note, that the frame's geometric tolerance applies to the complete set of repeated features as a single pattern, not to each feature in isolation with its own dimensional tolerance. All the pattern's features share the same datum system and the same theoretical tolerance zone.

At the feature's maximum material condition (MMC), the allowed geometric deviation is exactly zero: the feature must be perfectly at the theoretical position when at its most unfavorable size limit. The available geometric tolerance only appears (grows) as the feature's real size moves away from MMC toward LMC — hence the "0" next to the symbol in the frame, instead of a fixed value.

Controls, in a specific cross-section, the total variation (combining form and position) of a circular feature or a face when rotated 360° about the datum axis. Unlike roundness (which requires no datum), circular runout does depend on the clamping about the reference axis: eccentricity, ovality and misalignment blend into a single reading, without distinguishing which of the three causes the error.

Controls the combined variation of form, orientation and position of an entire surface (not just one section) when rotating about the datum axis, with the indicator sweeping the whole length or face simultaneously while the part turns. It's more demanding than circular runout because it accumulates the error of all sections into a single reading: a part can pass circular runout section by section and still fail total runout if the error pattern changes progressively along the part (for example, a slight taper).

Controls the form of a complete surface (not just one section) against the three-dimensional theoretical model (CAD or basic dimensions), and can replace several form, orientation and position tolerances in a single requirement if the drawing states it explicitly (profile as a composite tolerance). The tolerance zone is an envelope of constant thickness around the whole nominal surface.

Controls the form of a line (the contour seen in a specific cross-section) against the theoretical profile defined by basic dimensions or by the CAD model, without comparing the whole 3D surface (that's what profile of a surface is for). The tolerance zone is a band following the theoretical contour, of width equal to the stated value; it can be distributed symmetrically on both sides of the nominal profile or unequally (unequally disposed, with the ⊕/⊖ modifier) toward one side.

Extends the scope of a profile tolerance (of a line or a surface) so it automatically applies to the WHOLE closed contour shown in that view, without dimensioning each segment separately. It's indicated with a small circle on the corner of the tolerance frame ("all around"). It differs from applying profile only to a segment marked with a thick line: without the circle, the profile only controls the explicitly indicated segment.

Straightness applied to the derived axis (the theoretical center) of a cylindrical feature, not to its surface. With ⌀ before the value, the tolerance zone is an imaginary cylinder of that diameter within which the real axis must stay over its whole length — not a pair of planes as in line straightness. It's the correct way to control the complete axis of a shank or stud; being a feature of size, it's the only straightness that accepts Ⓜ or Ⓛ.