Or Grammage. The weight per unit of paper, measured in grams per square metre. It is worth remembering, if you buy a lot paper, that it is a measurement of weight, not thickness or strength. A Kraft, which is relatively dense and with long strong fibres, will be thinner than a bulkier, less dense Art of the same GSM. The Kraft is likely to be stronger, but the Art may well exhibit more bulk and be more rigid.