Lean is nothing more than what direction a stitch slants. Knit and purl stitches are neutral, meaning they do not slant. When we look at a section of fabric made purely of knit and purls everything is in nice neat parallel columns of stitches.
Now I am sure you are asking yourself why are we talking about neutral when this article is about decreases? To appreciate and truly read your knitting we have to be able to know what neutral looks like in order to see what the lean of a decrease looks like. Some are very blatant while at other times they are quite subtle and will blend in quite nicely.
When it comes to decreases we have many types. The most common decreases encountered are the basics which take two stitches and combine them into one as when using Knit Two Together (K2Tog), Purl Two Together (P2Tog), Slip-Slip-Knit (SSK), Slip-Knit-Pass (SKP), Knit-Slip-Pass (KSP), Knit Two Together Through the Back Loop (K2Togtbl), Purl Two Together Through the Back Loop (P2Togtbl), Slip-Slip-Purl Two Together Through the Back Loop). Keep in the back of your mind you can also encounter Knit Three Together (K3Tog), Knit Four Together (K4Tog) etc. Essentially when you see K#Together, with # being the actual number of stitches you will be combing you are performing a decrease. With any of these decreases the number can be increased and that is what you will use. Enough about the technical side and let's get to the good stuff... lean!
Let's begin with the two most basic decreases the K2Tog and the SSK.
In the image below is a piece of fabric knitted right handed. On the left side of the fabric is an SSK. For a right handed knitter an SSK is a left leaning decrease ( \ ). On the right side of the fabric is a K2Tog. The K2Tog for a right handed knitter is a right leaning increase ( / ). Notice how, because of their placement on their respective sides, they appear to flare outward while the fabric is decreasing inward. This effect is also known as full fashioned.
For those who learned with a strong English Knitting background the Slip-Knit-Pass (SKP) and the Knit-Slip-Pass (KSP) are the decreases encountered more commonly than the K2Tog and the SSK. These decreases are equivalents regarding their lean. The K2TOG leans the same as the KSP and the SSK leans the same as the SKP (see below). Now that we know what is equivalent to what we can then look to see if a pattern has SKP and KSP in it we can then see we simply swap the two for their placement, just like we did with the K2Tog and the SSK, to end up with the desired lean or we can substitute a different decrease with the appropriate lean.
Lastly, a few words on purl side decreases. These are exactly as they sound; decreases made on the purl side of the fabric however the resulting lean is visible on the KNIT side of the fabric. The three a knitter is most likely to encountered are the Purl Two Together (P2Tog) , the Slip-Slip-Purl (SSP) and the Purl Two Together Through the Back Loop (P2Togtbl) The P2Tog is a quite common general use decrease. The SSP and the P2Togtbl not as common and are most likely to come up in lace patterns. Purl side decreases, aside from P2Tog, are not used anywhere near as often as knit side decreases, so I am not going to go into too much detail. I will leave the detail for a future blog. Just keep in mind that they do exist AND produce lean on the right side of the fabric.
When it comes to decreases the best advice I can give you is to take some time and experiment. Pick a decrease, make a stockinette swatch and make the decreases. Look at the right side of your work and observe how the decrease leans, flip your work over and see the wrong side looks like and make a note of both. The more frequently you examine your work, how the stitches are formed and how the completed stitch looks the more you will understand what you have created and how to use them in various ways.