The most common form is a block swap where members make a set number of the same block usually with a specific design, theme or type of fabric.
But swaps are not limited to just blocks because any project can be swapped.
2019...Quilts Across the Pacific
An exchange of miniature quilts between quilters in San Jose in California USA... &... Newcastle N.S.W. Australia.
Just one of the beautiful exchange quilts from the USA... and...
you can see ALL the quilts... HeRe.
You can read all the swap details... HeRe.
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2014... Civil War Churn Dash Swap - 41/2in Block... Organised by Cathy Pitkin.