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Purl through back loop is a twisted stitch. It can be a little tricky to make and may seem counterintuitive in the way it is done. The thing to remember is that you want to twist the stitch, not open it up.

Bring the right hand needle to the back and around the stitch, insert the right hand needle through the back loop from left to right and put it in the purl position with the tip of the right hand needle pointing left and towards you, wrap yarn around the right hand needle and pull it through sliding the worked stitch off the left hand needle and leaving the new stitch of the right hand needle.

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