You asked for it, here it is.
If you look at the shape of the spine
(scroll down to the second illustration) from the side, you can see that where the neck vertebrae and the thoracic vertebrae come together it curves to the back. The curvature of the spine is designed to handle the stresses, motions, and loads we place on it every day. The particular curve to the back at the base of the neck is what makes back neck shaping in a garment important. The vast majority of necklines will hit at this curve on the back of the neck. If the garment doesn't have some shaping along the back neckline (in other words, it goes straight across from shoulder to shoulder), the garment will fall backward to fit around the back neck curve. This is most noticeable in garments with larger front neck line shapes such as V necks, scoop necks and cardigans, which may or may not have closures, but are typically not fastened all the way up even when they do. We've all worn garments that fall back at the neckline and we spend all day pulling them forward, back into place. If the front neckline is a jewel neck, also a typical T shirt round neck, and the back neckline has no shaping, we won't spend the day pulling the garment forward so much as we'll spend the day feeling like our clothes are strangling us, because the smaller neckline area can't really go far enough to compensate for our natural spinal curve.
If you look at traditional folk knitwear you will find many examples of garments made without back neck shaping. We've learned a lot about shaping garments to the human body since then and have found that adding some back neck shaping is much more comfortable and flattering to wear. Now each designer has their own approach, and as I was looking through my books to figure out where I learned about back neck shaping and who's technique I was using (several places, and my own), I did find a note that back neck shaping can be avoided when you have a large collar. The theory behind that is probably that if you're going to add on all the extra for the collar, you'll eliminate any back neck shaping. I personally disagree, but this is my own personal feeling on the matter. Even on a garment with a hood (pretty much the biggest collar you can have), by adding back neck shaping, the hood itself will adapt to the curve of the spine. On the Minimalist Cardigan, there is a large collar that goes all the way down the front to the hem. This might
, all by itself keep the cardi in place. I don't feel like taking the chance (because I hate clothes I have to fidget with and I'm too lazy to go back and redo a neckline after finishing the sweater), so I added back neck shaping.
I found a couple of books that dealt with back neck shaping and they both gave rules for figuring it out. Naturally, both of the rules were different, and since I figured the back neck shaping all by myself without looking it up, I made up my own rules. This just goes to prove that back neck shaping, like pretty much everything else in knitting can be done more than one way, so I'm not going to give you rules, I'm going to give you guidelines. If you've never changed anything about a pattern before, this is a good place to start - it doesn't take too much time, isn't too math intensive, and if you have long hair and really mess it up, no one will know. All you'll need is some paper and a pencil to make some notes with and a calculator if you don't trust your math. It doesn't take very long, so you don't even have to send the whole family out to pick up dinner (unless you want to, and really, that's not ever a bad idea when you get down to it).
The first thing you need to do is look at your pattern and figure out how many stitches you have after the last arm hole decreases are worked (these can be actual decreases or bind offs, depending on the pattern - just look for the stitch counts which are usually right before the sentence, "Continue working even until armhole measures X inches." If you've already begun your armholes, this is fine, but only work even until your armhole measures 1/2 of the X inches the pattern calls for before shoulder shaping, and then you can double check that your stitch count matches up with the pattern's.
The next thing you need to figure out is how many stitches are used in the shoulders (remember, there are two shoulders) and how many are used in the back neck. You should be able to say (2 x shoulder stitch count) + back neck stitch count = total stitch count after armhole decreases are completed. If this isn't adding up properly, double check your math and the pattern's math until you figure out for sure how many stitches go where. For the back neck shaping, you'll only be dealing with the back neck stitches so you have to know how many of them there are.
OK, here's where the guidelines come in. Your back neck shaping will usually be anywhere from 1" to 1/2" from the top of the shoulder shaping or final shoulder bind off. Most shoulder shaping takes up about 1/2" of the back length, so if you start your back neck shaping at the same time as the shoulder shaping, you'll end up with a back neck depth of about 1/2". I usually use 1" for my own back neck shaping because it gives me a little more spine space and means I definitely won't have to fidget with my garment. I don't really know of a nice way to say this, so I'll just say it: if you have a hunchback type of shape (I'm assuming if you do, you know this), you will want to have a deeper back neck shape than 1". This type of spine shape is more common among older women, but we are all different shapes and sizes. If you're honest with yourself about your own shape and size, you'll have better fitting garments and you'll look better than if you just pretend you don't have a bump on your back.
So, now you know how many stitches your back neckline will use in total and you know the depth your back neckline will be. Now we have to shape it. (You could theoretically just bind off the total neckline stitches at the correct depth and then continue working the shoulders as the pattern directs, but now you're putting a curve in a rectangular opening and if you're planning on picking up stitches to add a collar band of some sort later, you'll be unhappy with the way it goes around the corners. You'll be happier if you slope the back neck at least a little bit.) Since you know the depth of your back neck shaping, figure out how many rows you've got to use. This is where it's really nice if you've actually knit most of the back of the sweater, because you can measure the actual garment's row gauge instead of relying on a gauge swatch (gauge swatches lie sometimes). If you're getting 8 rows per inch and you're planning on a back neck depth of 1 inch, you've got 8 rows to do your back neck shaping in. If you're planning on a back neck depth of 1/2 inch, you've got 4 rows to do your back neck shaping in - see? Easy math!
I tend to write down the exact number of rows I have to work with in a list and then mark WS or RS so I can keep track of things while I figure the shaping. I like to bind off 75-80% of the total back neck stitches on the first row (make sure you bind off the center 75-80%, or your neck shaping won't be even). This is not set in stone of course - guidelines, remember. Then you have a couple of options. If you're working a shallower back neck (1/2 inch), you need to get the rest of the back neck stitches decreased pretty quickly. In that case, I might work a WS row even and then bind off the remaining stitches on each side of the center on the following two rows. If you're working a back neck of 1 inch deep, you can work your back neck exactly as I described above and actually work a few rows of shoulder stitches only (all the back neck stitches will be bound off by row 4) or you can work decreases at the neck edge for a few rows and spread your back neck shaping over more rows. (Don't forget, you can also combine decreases and bind offs - the possibilities are limitless, as long as you don't decrease or bind off more than the total number of stitches from the back neck.) Note that all theoretical row numbers used in the above examples go with the theoretical 8 rows per inch I used as an example in the previous paragraph - you have to use your own numbers because your sweater might be different.
The other thing to remember when you're doing this is that usually you will be working shoulder shaping for at least part of the time you're working back neck shaping (that dreaded instruction, "At the same time..." is important here), so make sure you don't get so wrapped up in your back neck shaping that you forget your shoulder shaping.
The first time you figure this out for a pattern, it might help to look at another pattern that does have back neck shaping and compare it to my notes, then you can see how back neck shaping is done with real numbers. It's not hard to do and if you sit down with your knitting and just follow the guidelines, you'll probably do just fine. The limited space involved in back neck shaping really cuts down on the amount of mistakes you can make, so go for it!
The two books I was able to find that discuss back neck shaping (and I know there's more, I just grabbed these two) were Deborah Newton's Designing Knitwear
and the big Vogue Knitting
book. Both discuss more design than just back neck shaping and if you find yourself tinkering with patterns a lot, or want to design your own sweater from scratch, you'll find them helpful. I personally prefer Newton's for just design instruction, but the Vogue Knitting is also good and covers a wider variety of subjects.