The centred double decrease is a lovely, prominent decrease that turns 3 stitches into 1. It creates a clear strong line when the decreases are stacked one on top of the other as shown above.  Most people are familar with how to do the CDD with knit stitches. Here we explore executing the CDD from the purl side. Both the centered (or central) double decrease from the knit side and from the purl side are used in the Adventus Scarf and Wrap pattern for mini-skein advent calendars from 2.5g per skein and up.

On the right side (knit side) the centered double decrease is done simply enough: slip two stitches together as if to knit (this switches their position, making the second – or middle stitch – the right most stitch), the next stitch is knit and then the slipped stitches are passed over this knit stitch. This brings the centre stitch to the top, standing straight up over the stitches below it. The other two stitches come together hiding their slants behind this straight stitch.

Creating the centered double decrease on the purl side is just a touch trickier, but there are several ways of approaching it. I recommend trying them all out and seeing which is most comfortable for you.

Orient the stitches Work the stitches
SSP variation (see video below). Uses the same method as doing an ssp. Slip the first stitch as if to knit, slip the next stitch as if to knit. Return the two stitches to the left needle (by slipping them purlwise – slip the left needle into the two stitches from left to right). Slip both stitches together through the back loop. (To do that, move the right needle behind the left needle and enter the two stitches from the left to right at the back of the work and slip them off the left needle.) Purl the next stitch. Pass the two slipped stitches over the knit stitch.
Cable variation (see video below). Uses the same method as a 1 x 1 cable (twisted stitch). Go into the second stitch on the left needle as if to purl from the back. Pinch both stitches against the right needle with the right thumb, slip both stitches off the left needle, and immediately pick up the loose (first) stitch with the left needle. Purl the next stitch. Pass the two slipped stitches over the knit stitch.
Laura Nelkin’s MethodSlip the second stitch on the left needle, slip it off the eddel and replace the first stitch back onto the left needle. P2tog Pass the slipped stitch over.
Fleegle’s Method Slip the first stitch as if to knit. Slip the next stitch as if to knit Return the two stitches to the left needle by going into the first stitch then the second stitch from right to left (a mirror of the knit 2 together).Purl all three stitches together.

The video below shows more details and information on the other stitches needed for the Adventus Scarf and Wrap.