When patterns call for decreases, they don’t always specify which kind. What’s with all the different types, and how do you know which to use when?
The most common decreases join two stitches together to decrease the stitch count by one. All two-into-one decreases lean, either to the left or to the right. This is because when you make the decrease, two stitches are pulled together to meet in the center, with the left one leaning toward the right and the right one leaning toward the left. They end up one on top of the other, not side by side, and how you execute the decrease determines which stitch ends up on top. Since the upper one is the one you see, it determines the direction in which the decrease appears to lean.
This matters because it affects the appearance of knitting. Decreases are inconspicuous when they lean the same way as the edge of your knitting. For example, if you are working a top-down sock and are decreasing foot stitches to shape a wedge toe, the left side of the toe curves in toward the right. A right-leaning decrease will lean in the same direction as the toe and will not be very noticeable. Because of this, “paired decreases” are often used in these situations: a left-leaning decrease on the left-leaning edge, and a right-leaning decrease on the right-leaning edge. If there is no edge - for example, when decreasing across a row to shape the crown of a hat that is worked in the round - you can use any decrease you want. In this case I usually choose the k2tog decrease, because it’s fast and easy to do and it looks good.
The k2tog is the most common right-leaning decrease. The most common left-leaning decreases are the SKP and SSK, and you can substitute one for the other since they lean the same way. The SKP is used more in older patterns, and stands for “slip one stitch knitwise, knit one stitch, then pass the slipped stitch over the knit stitch.” The SSK is more common in newer patterns, and I think it looks tidier than the SKP (but try it for yourself). It stands for “slip one stitch knitwise, slip another stitch knitwise, then slide the left needle into the fronts of the stitches, knitting them together through their back loops.” Note that although normally you slip stitches purlwise in knitting, you slip them knitwise for decreases; this is to prevent twisted stitches. There is an SSK variation in which the second stitch is slipped purlwise; this twists the bottom stitch (after you knit through their back loops) and makes the decrease a little tighter and tidier, and the twist is not very noticeable since it’s under another stitch. You can learn about other types of decreases in any knitting reference book.
One tip to improve the look of all decreases is to work them on the very tips of the needles, where the needles are thinnest. Pushing the needle through the stitches far enough to slide them onto the thickest part of the needle usually stretches them out, and they end up looser and looking more noticeable. Paying attention to this will make your knitting look more professional.