Metric ruler marks

A long metric/imperial thread on the Australian Woodworking forum included a discussion on the best way of marking a metric tape. The fact the the marks at 5mm were slightly longer than the other 1mm marks (but shorter than the 10mm marks) was considered a plus point. I have just bought a Starrett 6"/150mm metric conbination square. I was surprised to find that there are four heights of marking (longest to shortest): 10mm, 5mm, (2,3,7,8)mm, (1,4,6,9)mm. Or, IOW, extra prominence is given to the markings at 2,3,7,8 mm position within 10mm.
Anyone know why? I have the feeling that they would have preferred to have marked 2.5 and 7.5, following imperial practice! Even odder, two of the four edges have 1/2 mm as unit, so markings at 2.5 and 7.5 would have made some sense there! (and please, no wars. . .)
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Sounds reasonable. It uses the exact same reasoning as making the 5mm mark prominent. How else would you do it? I don't consider this an "imperial vs. metric" issue. Really it is just a way to subdivide something into 10 rather than 8 or 16. You would have the same issue with 10'th of inches.
-j
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