A legacy numbering system for sheet thickness, wire diameter and drill size.
Gauge numbers are historical and each series is independent: 16 gauge sheet metal is 0.0598 in in the standard steel series but 0.0508 in in galvanised, and wire gauge runs the opposite way with larger numbers meaning thinner wire.
They survive because they are short and familiar, but every gauge number must be converted to an actual dimension before it means anything.
Assuming a gauge number means the same thickness across materials. Steel, galvanised steel, aluminium and stainless all have different gauge tables.
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