There is nothing particularly mathematical about The Woolsack. It is a tessellation of 12 large squares, giving a hint of other shadowy squares between them. It’s hidden secret is that it actually has 13 squares and can fold into itself to make cushion.
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Each of the large squares is made separately, in one piece, using one ball of yarn at a time. Twelve squares are stitched together, as shown in the photo. The 13th square is stitched on the back to form a pocket so it can turn into a cushion.
This afghan is intentionally scruffy but it is possible to make variations that look much tidier. It is a wonderful way to use up remnants of yarn. Any lengths can be incorporated. The original was made in Double Knitting yarns but could be in any other type of yarn. It will be of a more uniform texture if you use approximately the same weight of yarn throughout .