DECREASING:

K2TOG (Knitting Two Stitches Together) & SSK (Slip, Slip, Knit)
Decreasing is another methods of shaping a knitting piece. There are several methods of decreasing and here are just two.

K2TOG

The most common way of doing this is to knit 2 stitches together or purl 2 stitches together. The method you choose depends on what stitching pattern you are using. If the stitches would normally be knitted, then knit 2 together to decrease. If the stitches would normally be purled, then decrease by purling 2 together (p2tog).

To knit 2 stitches together, just insert the right needle into two stitches at the same time and knit. This will result in a decrease of one stitch. This decrease will slant to the right.

SSK

For this method of decrease, slip the first stitch as if to knit,

then slip the next stitch as if to knit.
Insert the left needle into the front of both slipped stitches

and knit them together.

This decrease slants to the left.