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Slip 2 and purl together through back loops. This is a single left-leaning decrease on the purl side. It's a tidy stitch, but a bit tricky due to the back loop purl.

With the yarn held in front slip two stitches individually knitwise from the left hand needle to the right hand needle, move both stitches back onto the left hand needle and purl both through the back loops.

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