This term ("afterthought thumb") was coined many years ago by renowned knitting author and teacher, Elizabeth Zimmermann. Today we have changed it slightly by marking the area to be unraveled with a contrasting waste yarn.
The advantage to this technique is that it allows the knitter to continue working even without worrying about shaping. It is a good choice for heavily patterned projects, such as Fair Isle. Instead of stopping to shape the thumb, you simply mark the stitches by working them in waste yarn and come back to them later. When it’s time to work the thumb, snip one stitch in the waste yarn row and carefully unravel it, removing the waste yarn. Live stitches will be revealed above and below the waste yarn. Divide the stitches evenly on the needles, join yarn, and work in the round, decreasing as directed in the pattern before grafting the last bit closed using Kitchener stitch.
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