Of the three basic diameters that describe a screw thread, the pitch diameter E is the working one: the diameter at which the thread groove and ridge are equally wide, where the flanks actually engage and loads transfer between bolt and nut. Thread gauging, class fits (2A/2B, 3A/3B), and tap-drill engagement math all operate at the pitch diameter — the reason ASME B1.1 tabulates it to more decimal places than anything else.
For the Unified 60° form its position is fixed by geometry: E = D − 0.649519 × P below the basic major diameter, with P = 1/TPI and 0.6495 being three-quarters of the 0.866·P fundamental triangle height. A 1/4-20 UNC thread has a 0.2500 in major and a 0.2175 in pitch diameter. The same quantity threads through strength work: the FED-STD-H28 shear-area formulas evaluate the basic pitch diameter as Es = D − 0.6495·P, and the tensile stress area a bolt breaks across is computed on an effective diameter between the pitch and minor diameters. All tabulated values are basic dimensions — actual product threads carry class allowances and tolerances below basic.