The pitch diameter (PD, or "D"2) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points, when viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance "D"2 away from it, the "PD line," slices the "sharp-V" form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major ("D") and minor ("D"1) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, "D"2, "D", & "D"1, together, would fully describe the thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement "D" − "D"2 away from the position of the PD line.