Why sail cloth is lighter in USA?

Every now and then somebody posts to rec.kites a question like:

I cannot find the conversion from oz. to gr./square meter for ripstop. Someone said me that 0.75 oz. is equivalent to 42 gr./sm. but this conversion do not always work.

The answer is: 

For historical reasons a "square yard" of *sail* 
cloth in US is only 28.5 inches wide. You need to 
know whether the "oz" weight is from US or from 
UK. 





Ref: Jeremy Howard-Williams, Sails, Adlard Coles 
     Ltd, London, ISBN 0 229 11570 5, 
     fourth edition, page 55.

BEGIN QUOTE --------------------------------------

The Americans measure sailcloth in ounces per yard 
of cloth 28 1/2 in. wide, whereas in Britain the 
square yard is used. There is thus an apparent 
difference of 20 percent in our respective cloth 
weights, for a yard of cloth in Britain will have 
more material to be weighed (being 36 in. wide) 
than a yard in America (being only 28 1/2 in 
wide). 

END QUOTE ----------------------------------------

Conversion between US / UK / ISO units: 

US oz/(yd x 28.5 inch)       UK oz/yd2       g/m2
<---------------- x1.25 ---->         <-- x34 -->
<------------------------------ x42 ------------>

Multiply/divide by the x-number, e.g. Carrington 1.8 oz/yd2(UK):

   1.8 x 34 -> 62 g/m2
   1.8 / 1.25 -> 11/2 oz(US)