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Allowance: The minimum clearance or maximum interference which is intended between mating parts. Angle of Thread: The angle included between the flanks of a thread measured in an axial plane. Back Taper: A slight taper on the threaded portion of the tap making the pitch diameter near the shank smaller than that at the chamfer. Basic: The theoretical or nominal standard size from which all variations are made. Chamfer: The tapered and relieved cutting teeth at the front end of the threaded section. Common types of chamfer are taper, 8 to 10 pitches long, plug, 3 to 5 pitches and bottoming, 1 to 2 pitches. Crest: The top surface joining the two sides or flanks of a thread. Cutting Face: the leading side of the land. Flute: The longitudinal channels formed on a tap to create cutting edges on the thread profile. Heel: The following side of the land. Height of Thread: In profile, distance between crest and bottom section of thread measured normal to the axis. Hook Face: A concave cutting face of the land. This may be varied for different materials and conditions. Interrupted thread: Alternate teeth are removed in the thread helix on a tap; usually restricted to those having an odd number of flutes. Land: One of the threaded sections between the flutes of a tap. Lead of Thread: The distance a screw thread advances axially in one turn. Major Diameter: The largest diameter of the screw or nut on a straight screw thread. Minor Diameter: The smallest diameter of the screw or nut on a straight screw thread. Neck: The reduced diameter, on some taps, between the threaded portion and the shank. Pitch: The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis. Pitch Diameter: On a straight screw thread, the diameter of an imaginary cylinder where the width of the thread and the width of the space between threads is equal.		Point Diameter: The diameter at the leading end of the chamfered portion. Radial: The straight face of a land, the plane of which passes through the axis of the tap. Rake: The angle of the cutting face of the land in relation to an axial plane intersecting the cutting face at the major diameter. Relief: The removal of metal behind the cutting edge to provide clearance between the part being threaded and a portion of the threaded land. Also, see back taper. CHAMFER RELIEF: The gradual decrease in land height from cutting edge to heel on the chamfered portion of the tap land to provide radial clearance for the cutting edge. CON-ECCENTRIC RELIEF: Radial relief in the thread form starting at the back of a concentric margin. ECCENTRIC THREAD RELIEF: Radial relief in the thread form starting at the cutting edge and continuing to the heel. Root: The bottom surface joining the flanks of two adjacent threads. Side or flank of thread: The surface of the thread which connects the crest with the root. Shank: The portion of the tap by which it is held and driven. Spiral Point: An oblique cutting edge ground into the lands to provide a shear cutting action on the first few threads. Square: The squared end of the tap shank. Thread: The helical formed tooth of the tap which produces the thread in a tapped hole. Thread Lead Angle: The angle made by the helix of the thread at the pitch diameter, with a plane perpendicular to the axis. Threads Per Inch: The number of threads in one inch of length. Thread: SINGLE: A thread in which lead is equal to pitch. DOUBLE: A thread in which lead is equal to twice the pitch. TRIPLE: A thread in which lead is equal to triple the pitch.