Earlier, karel wrote:
...for those unawa metric screws, bolts &C only have a diameter and a length
M4*40 means it is 40 mm long and will pass a 4mm hole,
now isn't that all there is to say?

Um, no, not quite. Like inch-size fasteners, metric fasteners will
often have more than one thread pitch available. I know I've spent more
than a little time sorting M10 nuts into 1.25mm and 1.5mm thread
pitches.