PCDD (purl, central double decreas) is a double decrease worked on the purl side of the fabric, that decreases one stitch either side of a central stitch, decreasing your total number of stitches by two.
How to work a PCDD decrease
So I have cast on 11 stitches and worked a few rows in stocking stitch, on the next purl row the instructions are :
P4, PCDD, purl to end. (8 sts)
I have purled the first 4 stitches of the row and am now ready to work PCDD:
Step 1 – Slip 1 stitch knitwise from the left hand needle to the right hand needle, slip the next stitch knitwise in the same way (you now have 2 slipped stitches on the right hand needle).
Step 2 – Pass both slipped stitches back on to the left hand needle by inserting left hand needle (from left to right) through both stitches.
Step 3 – Slip 2 stitches together through the back loops.
Step 4 – Pass the 2 slipped stitches back onto the left hand needle once again.
Step 5 – Purl 3 stitches together; place tip of right hand needle through the first 3 stitches on left hand needle and wrap yarn round counter clockwise (as you would if you were doing a normal purl stitch), pull yarn through dropping the 3 stitches off the left hand needle.
A central double descrease on the purl side completed.
On the above sample I have worked a few more rows with a PCDD decrease in the middle of each purl row. You can see how this decrease causes the left and right edges of the fabric to slant evenly in towards the middle.