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Glossary
Nominal Diameter
Fasteners / threads: Nominal diameter is the named size of the fastener, and for external threads it corresponds to the basic (nominal) major diameter used in the thread designation (e.g., M10 → 10 mm; 1/4-20 → 0.250 in). The actual measured major diameter can vary within the thread tolerance, but the “nominal” is the size label.
Pipe / tubing: Nominal pipe size (NPS/DN) is a size designation, not the actual OD (and not necessarily the actual ID either). In most cases the pipe OD is fixed for a given NPS, while the ID changes with schedule/wall thickness, which is why “nominal” in piping is essentially a trade size.