It is necessary to have a cornhole bag for your cornhole game because that is what you need to score points. Cornhole bags are like beanbags, and that is what you use to throw into the hole at the farthest board. If you didn't have the cornhole bags, you wouldn't be able to play this game.
Piehole
Cornhole bags are what most call bean bags. They are made of duck canvas and are filled with dried corn. Cornhole bags are used mostly in lawn games where the players stand on platforms that have holes in them.
It is 6 inches in diameter located in the center of the board and 9 inches from the top of the board to the center of the hole. For cornhole board design ideas check out: http://www.cornholeace.com/cornhole-board-designs.php
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more then a few
There are several different options available online in order to find out how to sew cornhole bags. One site would be for example "Cornhole-How-To". Another one is the guide at "Big Duck Canvas Warehouse" entitled "How To Make The Best Cornhole Bags".
21 Feet
Twelve or 16
how to clean cornhole bags
Things like cornhole, ladderball, and softball
God shoves corn up your cornhole