CDD (central double decreas) is a double decrease that decreases one stitch either side of a central stitch, decreasing your total number of stitches by two.
How to work a CDD decrease
So I have cast on 11 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :
K4, CDD, knit to end. (8 sts)
I have knit the first 4 stitches of the row and am now ready to work CDD:
Step 1 – Slip 2 stitches together knitwise from the left hand needle to the right hand needle.
Step 2 – Knit the next stitch on the left hand needle.
Step 3 – Pass the 2 slipped stitches (together) over the knit stitch – inserting left hand needle through the slipped stitches from left to right and lifting them over the knit stitch.
A central double descrease completed.
On the above sample I have worked a few more rows with a CDD decrease in the middle of each knit row. You can see how this decrease causes the left and right edges of the fabric to slant evenly in towards the middle.