Dec is the abbreviation for decrease, which is how you remove stitches. There are many ways to decrease and you can use the method you prefer, unless the pattern specifies a specific decrease to be used. In that case, use the decrease indicated.
Decreases can be right-leaning or left-leaning, single decreases or double decreases. As a default, use the easiest method, unless a particular method of decrease is specified.
Slip stitch just worked back to left needle and pass next stitch over slip stitch. Decreases 1 stitch.
Slip 2 stitches to right needle. *Use left needle to bring second stitch over the first stitch, dropping second stitch off both needles; slip remaining stitch from right needle to left needle and use right needle to bring second stitch over the first stitch, dropping it off both needles. Slip remaining stitch to right needle. Decreases 2 stitches.
Slip 2 stitches to right needle. *Use left needle to bring second stitch over first stitch and drop from needle;* slip first stitch to left ndl and rep * to *. Slip first st to right needle. Decreases 2 stitches.
DD – double decrease
SSK, return new stitch to left needle and pass next stitch over new stitch, then return new stitch to right needle.
This decrease is worked on a wrong side row. Slip 3 stitches to right needle. Use left needle to lift the second stitch over the first stitch and drop it off the needle; repeat. Slip the remaining stitch back to the left needle and use the right needle to lift the second stitch over the first stitch; repeat. K1 in remaining stitch. Decreases 5 stitches to 1 stitch
Slip 1, use right needle to pass third stitch over first two stitches and off the left needle a total of 10 times, then knit first two stitches together and pass the sl1 over the k2tog. Decreases 12 stitches.