A decrease in knitting is a reduction in the number of stitches, usually accomplished by suspending the stitch to be decreased from another existing stitch or by knitting it together with another stitch. When more than one stitch is suspended from a stitch, they can hang in different orders. For example, the first stitch could be on top of the second stitch (when seen from the right side) or the reverse, leaning to the left or the right.
The order of stitches is important, both for appearance and for the way it pulls the fabric. Sometimes a double decrease is made, in which three stitches are suspended from a single stitch. This allows for six possible stitch orders: 123, 132, 213, 231, 312 and 321.
Here, the first number is the topmost stitch, and the last number is the bottommost stitch. Thus, 213 means that the second stitch is uppermost (as seen from the right side), followed by the first, then third stitches. The uppermost stitch is most important; there is not much visual difference between 213 and 231.