When a designer writes a pattern and the pattern indicates a decrease the pattern generally specifies the kind of decrease. The pattern will often say knit 2 together (k2tog) or slip, slip, knit (ssk or skp). These decreases are specified for a specific reason often, the reason being that they slant right and left and a right or left slant looks better. Learning which decrease slants in which direction will make your knitting look better, more finished and will help you fill in the blanks if a designer doesn’t specify what kind of decrease to use.
A knit 2 together, also abbreviated as k2tog, decrease is when you insert the right hand needle through the front to the back (as if to knit) of the next two stitches on the left hand needle simultaneously and knit them together. This decrease slants to the right.
If you knit 2 together through the back of the loops or k2tog tbl, you insert your right hand needle through the back of the next two stitches on the left hand needle, wrap your yarn around the right hand (as if to knit) and knit them together, this will cause the stitch to slant to the left.
There is also a decrease called an SSK, that is a left slanting decrease. In this decrease you slip the next two stitches as if to knit-one at a time- then knit them together.
Another decrease I’d like to mention is a variation of the SSK, which is that you slip the first stitch on the left hand needle as if to purl, then the next stitch on the left hand needle as if to knit, finally you knit them together knitwise.
A final decrease I’d like to add is a PSSO or pass slipped stitch over. In this decrease when you come to the spot to decrease you slip the stitch from the left needle to the right needle as if to purl, knit the next stitch, then pass the slipped stitch over the top of the knit stitch.