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Purl 1 into front and back of a stitch is typically a single stitch increase unless the pattern specifies. Purling front and back is a little tricky and leaves clearly defined ridges where the increases are made. This is especially true when increasing by more than one stitch.

To do purl front and back, insert your right hand needle into a stitch and purl, do not slide the stitch off the needle, bring the right hand needle around to the back of the stitch, insert the needle to purl through the back loop, wrap the yarn around the right hand needle and draw it through the stitch, continue purling through the front and back loops as many times as necessary, slide the worked stitch off the left hand needle leaving the new stitches on the right hand needle.

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