I'll jump in with a little math to add to the good warrior's explanation.

If you have a clutch with a 12" friction area O.D. and a 10.5" I.D. (the friction material being 1 1/2" wide), then you multiply .7854 times 12 times 12 and find 113 square inches. Now multiply .7854 times 10.5 times 10.5 and find 86 square inches. Now, subtract 86 from 113 and find the actual lining area is 27 square inches.

If you have a clutch with a 11" friction area O.D. and a 9" I.D., (the friction material being 2" wide), then you multiply .7854 times 11 times 11 and find 95 square inches. Now multiply .7854 times 9 times 9 and find 63 square inches. Now, subtract 63 from 95 and find the actual lining area is 32 square inches.