A lifeline is a piece of waste yarn that allows you to rip back your stitches to the start at the beginning of a repeat when knitting lace.
Lifeline - What? Why? How? - 10 rows a day
Lifelines in Knitting – Elizabeth Smith Knits
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Knitted Lace Lifeline [A How To Video Tutorial]
How to Use Lifelines in Knitting (and Why You Need Them!) - Sheep and Stitch
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How To Knit: Lifelines — Video Tutorial – Brooklyn Tweed
Tutorial: beginner's lace, Blog
How to knit using a lifeline - The Blog - US/UK
First lifeline, now twisted stitches? : r/knitting
How to Use Lifelines in Knitting (and Why You Need Them!) - Sheep and Stitch
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How To Knit: Lifelines — Video Tutorial – Brooklyn Tweed