Non-nautical fabric weights are advertised per square yard (36" x 36"), often before coating.
Nautical fabric weights are per "Sailmaker's Yard", which is 28.5" x 36" or 0.792 square yards. Fabric that weighs 0.60 ounces per sailmaker's yard would weigh 0.76 ounces per square yard. (This is consistent with the MLD references made in posts above).
Then of course the SI units are different as well- one square meter would be 1.20 square yards or 1.51 sailmaker's yards. (I sure hope there isn't a "sailmaker's meter")
From Sailmaker.com:
sailmaker's yard:
An English/American unit of area for measuring the weight of sailcloth. It is 28.5 inches wide by 36 inches long. To say a piece of cloth is 6.5 ounce means a piece measuring 28.5" by 36" would weigh 6.5 ounces. This rather strange "yard" seems to be a remnant from the days when it was universally agreed that no one would ever be able to build a loom wider than 28.5 inches which would weave sail cloth. (Today 54 inches is common.)
Some other internet references mentioned an older width of 27".