**Major Diameter (d, D):**

The diameter of an imaginary cylinder (termed the majorcylinder) that would bound the crests of an external thread or the roots of an internalthread.

**Minor Diameter (d1, D1):**

The diameter of an imaginary cylinder (termed the minorcylinder) that would bound the roots of an external thread or the crests of an internalthread.

**Pitch (P):**

The distance, measured parallel to the axis, between corresponding points onadjacent thread forms in the same axial plane and on the same side of the axis.

**Number of grooves (n)**:

Number of grooves within one turn (n > 1 for a multistartthread).

**Lead ( l ):**

Pitch multiplied by the number of grooves within one turn (i.e. number ofstarts). Axial displacement of the screw after one turn. l = ⋅ Pn .

**Flank Angles (β, γ):**

The angles between the individual flanks (leading and trailing flank)and the perpendicular to the axis of the thread, measured in an axial plane section.

**Thread Angle (α):**

Sum of the two flank angles.

**Lead Angle (ψ)**:

Helix angle of the thread in the pitch diameter.

**Pitch Diameter (d2, D2)**:

The diameter of an imaginary cylinder (termed the pitchcylinder), the surface of which intersects the thread profile in such a manner as to makethe width of the thread ridge and the thread groove equal.

**Simple Pitch Diameter:**

The diameter of an imaginary cylinder, which intersects thesurface of the thread profile in such a manner as to make the width of the thread grooveequal to one-half of the basic (nominal) pitch.

**Virtual Pitch Diameter (Functional Diameter):**

The pitch diameter of an imaginarythread of perfect pitch and angle, cleared at the crests and roots but having the full depthof straight flanks, which would just assemble with the actual thread over a specified lengthof engagement.

This diameter includes the cumulative effect of variations in lead (pitch),flank angle, taper, straightness and roundness. For a theoretically perfect thread, the three definitions of pitch diameter define the samequantity.

The terms simple and virtual pitch diameter are used in gauging practice.Indicated value (m): Value of a quantity provided by a measuring instrument.