Metric thread sizes are specified differently than USA thread sizes. USA sizes are specified using the nominal major diameter and the number of threads per inch. For example, a 3/8-16 (coarse) thread has a 3/8" nominal major diameter and 16 threads per inch, while a 3/8-24 (fine) thread has the same nominal major diameter and 24 threads per inch.
Metric thread callouts are fundamentally different because the thread pitch (distance between each thread, in mm) is specified instead of threads per inch or per mm. For example, an M10x1.5 (coarse) thread has a 1.5mm pitch, while an M10x1.25 (fine) thread has a 1.25mm pitch.
To simplify metric thread callouts, the international (ISO) standard for metric threads eliminates the pitch callout on coarse threads. For example, the thread "M10" automatically means a coarse pitch, while any added pitch callout, such as M10x1.25, specifies a non-coarse pitch. This method of designation has the major benefit that average citizens do not need to memorize pitches just to identify a coarse thread.
Virtually all thread standards across the world use the ISO designation convention, including the German DIN and Japanese JIS standards. All except the ANSI standard, which so far still asks for a pitch callout on coarse threads, despite the confusion that it causes.
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