Decreases

Before we begin

Left- or right-leaning decreases are called left- or right-leaning because that is the way they look on the right-side of the work (RS). In particular, this refers to the inclination of the stitch that stays on top as seen from the RS. This is also true for double decreases. Also, in the case of the centered double decrease, it is the central stitch (of the 3 involved in this decrease) that stays on top when seen from the RS (this stitch does not lean in any direction).

Therefore, stitches which are worked on the wrong side of the work (WS), as described below, look exactly the same as their RS counterparts. In case you're wondering, all these stitches look very similar on the WS (see the photos on this post).

Another important difference between left- or right-leaning decreases is that they have the effect of pulling the fabric in their respective direction. This effect becomes very visible when decreases are stacked, i.e. when the same decrease is worked on every row or in alternating rows in the same position. Isolated left- or right-leaning decreases can be swapped without visible effect on the fabric.

Single decreases

Left-leaning

On RS:
  ssk, slip 2 stitches knitwise (one after the other), then knit slipped stitches together through the back loops.
  skp, slip 1 stitch knitwise, knit 2nd stitch, pass slipped stitch over knitted one.

On the WS:
  ssp, slip 2 stitches knitwise (one after the other), then bring them back (without twisting) to left needle, purl them through back loops.

Notes:

• ssk and skp are two different ways of working exactly the same stitch. Use whichever you prefer, even if the pattern calls for the other stitch.
• Another left-leaning decrease often used, k2togtbl (knit two together through back loop), creates a slightly different stitch, because it twists the two stitches. This stitch is easier to work but not as neat and, more importantly, it is not the mirror image of the standard right-leaning decrease (k2tog).
• There is another version of the ssk which twist only the stitch that is covered, often called improved ssk, which is worked as: slip the first stitch knitwise and the second stitch purlwise, then knit the two stitches together through the back loops.
• On the WS, there is an equivalent version of the skp, which I do not recommend because it's too much of a hassle in my opinion: purl one stitch, slip next stitch knitwise, bring both stitches back to left needle (without twisting), pass slipped stitch over 1st one, slip stitch back to right needle (without twisting).
• The equivalent of the k2togtbl on the WS is a p2togtbl (purl two together through back loop).
• The improved version of the ssp stitch is worked in the following way: slip the first stitch purlwise, the second stitch knitwise, then slip both stitches back to left needle and purl them together through the back loop.

Right-leaning

On the RS:
  k2tog, knit two together.

On the WS:
  p2tog, purl two together.

Note: although, just like for left-leaning decreases, you can make all kinds of variations by twisting 1 or 2 stitches, the simplicity of process and neatness of result of the above stitches means I never saw anyone doing that. 'Nough said.

Paired decreases

Often decreases are used in pairs in order to create shaping, for example, in the yoke of raglan sweaters knitted bottom-up or in sleeves knitted top-down. Usually a number of stitches are put between the decreases, although this is not strictly necessary. These are called guide stitch(es). Below a single stitch is used as illustration.

On the RS: ssk, k1, k2tog.

On the WS: p2tog, p1, ssp.

Note: for a different effect, the order of the decreases can be changed.

Double decreases

Instead of paired decreases, you can decrease two stitches in one go. This is most often used in lace patterns.

Centered or straight

On RS:
  s2kp (sl2togkw, k1, p2sso): slip two stitches together knitwise, knit next stitch, pass the two slipped stitches over knitted one.

On WS:
  slip two stitches knitwise (one after the other), bring stitches back to left needle, then slip them again, but this time together as if to purl through back loop. Finally, purl the next stitch and pass the two slipped stitches over purled one.

Left-leaning

On RS:
  sk2p (slkw, k2tog, psso): slip 1st stitch knitwise, knit together next two stitches, pass slipped stitch over knitted one.

On WS:
  purl two stitches together, slip next stitch knitwise, bring 2 sts back to left needle, pass slipped stitch over purled one, then slip that stitch back to right needle (without twisting it).

Right-leaning

On RS:
  k3tog, knit 3 stitches together.

On WS:
  p3tog: purl 3 stitches together.
  sp2p: slip 1st stitch knitwise, purl next 2 stitches together, pass slipped stitch over purled one.

Note: on most lace patterns, left and right leaning double decreases can often be interchanged, because their difference is not important to the end result. This means that patterns often used the stitch that is easier to work (in the author's opinion) and these stitches can be swapped as long as done consistently. In particular, left-leaning double decreases are often preferred on the RS, but right leaning double decreases are easier to work on WS. Note, however, that on lace patterns that include double decreases on both sides consistency is important.