To slip a stitch is to pass it from one needle to the other without working into it. Various methods
of decreasing (and many stitch patterns) require stitches to be slipped either knitwise or purlwise. The way you slip a stitch will affect the final appearance.
Slip one knitwise (sl1, Sl1, s1p, or S1p)
Slipping a stitch knitwise causes it to twist.
Insert the right needle into the stitch as if to knit it and slip it from the left needle onto the right.
Slip one purlwise (sl1p, Sl1p, or S1)
When a stitch is slipped purlwise, it remains untwisted.
lnsert the right needle into the stitch as if to purl it and slip it from the left needle onto the right.