The smallest diameter of a thread — at the root of the external thread, or the crest of the internal one.
External and internal threads have different minor diameters for the same nominal size, because the external thread has a rounded root and the internal thread a flat crest. Quoting one figure for both is a common error.
The minor diameter is what determines the bolt's tensile stress area and therefore its strength, which is why a fine thread — with a shallower thread depth — leaves a larger minor diameter and makes a stronger bolt at the same nominal size.
Using the external minor diameter for an internal thread (or vice versa). They differ by roughly 0.08 × pitch, which is enough to fail a gauge check.
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