Can someone explain to me how it became common to sell goban (and shogiban as well) that are 5mm short of a full sun multiple? For instance, I see boards listed as size #20 that are 5.5cm, size #30 that are 8.5cm, #40 that are 11.5cm, and so on. If you add 5mm of wood back to these boards, you basically get 2-sun, 3-sun, and 4-sun boards respectively. What's up with making boards 5mm short and calling them size #x0 where x is the sun size they fall just short of?

I have no idea. But that doesn't stop me from speculating.

I can't find a similar reference for Japanese standards, but in the US, hardwood is traditionally stated in inches unfinished. The finished size is smaller due to the finishing process.

http://en.wikipedia.org/wiki/Lumber#Hardwoods

For example, stated size of 4 inches is a final size of 3.75 inches.

5mm is approximately 0.20 inches, so the difference between stated size and finished size may very well be the same reason. (for reference, 0.25inches = 6.35mm)

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I imagine it's a bit like TVs and monitors, where you get, say, a 40" class TV that is actually 39.5, or a 22" monitor that is 21.5". It's cheaper to round, for them.

5mm doesn't sound like enough of an error margin for manufacturers to worry about too much.

As an afterthought, lumber sizes might also be a good point of comparison. 2"x4" beams not actually being 2x4 in cross-section and all that. I think that has to do with the original 2x4 having some of the wood removed when planing, etc.

At least here it depends on whether it is unfinished (sawn) or finished (planed). Unfinished 2" by 4" is pretty much 50mm x 100mm (I know that 2" is 50,8mm and 4" is 101,6mm, but since we are in the metric system here it is supposed to be 50x100 even if it's called "twobyfour" - "kakkosnelonen" in Finnish). Finished product is 45mm x 95mm. Similarly "twobytwo" is either 50x50 (sawn) or 45x45 (planed) etc.

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