Okay, okay. It's not fair to go around saying I'm working with Elizabeth to teach HER the magic of fitting your sweater and not share what I know with the rest of you.
The first step of any modification is to get accurate measurements. In this case, they're YOUR measurements. If you're human, you aren't really as symmetrical as most knitting patterns would have you believe. You'll also need to mark the center of the front of your sweater.
For bust dart measuring, you'll need three numbers (which will take four measurements):
1. The difference between the front length of your body and the back length of your body. (A-B vs A-C on the pretty pictures).
2. The distance from the top of your shoulder (A on the pictures) to your nipple (D) for top-down sweaters, and the distance from the hemline (B) to your nipple (D) for bottom up sweaters.
3. The distance between you bust points (nipples).
I'm not going to pretend that you'll know exactly what that meant, because even though some of you know, some of you won't. And what good is a tutorial that doesn't actually show you all the steps?
For the first number, you'll need two measurements: the distance from the top of your shoulder to your hemline down the front and back of your body. To take that measurement, you'll need to find the top of your shoulder. Ordinarily, I would say that you need that lovely point at the top of the shoulder where that little bump is --
(yes, I know -- exaggerated nipples -- but either my imaginary model is cold, or I've done it on purpose to make it impossible to miss where they are for measuring purposes). I'd say that you should just go straight in from that lovely knob (formed by the the acromio-clavicular joint) but I've been noticing that those of us who spend our lives knitting, and typing, and driving, and otherwise living in front of our bodies wind up rolling our shoulders forward ..
and thus using that knob will screw up your measurements unless you change your posture. (And yes, I know, I'm not the best artist in the world, so sue me.) (And yes, I'm likely to blog about that posture thing sometime soon too)
So, you start at the top of your shoulder straight up from the nipple (A), and measure snugly against your body to your hemline down your back (C). Then, you start at the same place (A), and measure snugly down your body to your hemline down the front of your body (B)over your bust-line (along the nipple line in fact (D)). For most of us, those numbers aren't the same.
Why snugly? Well, because that's how you'll find out the length your sweater will really need to be (unless your sweater falls straight down from your chest). And why at the nipple line? Because that's the longest line .... and you need to avoid going straight down from your shoulder, or you'll miss that part.
This difference is what we're accounting for when we put in bust darts. Assuming that the front measurement is longer than the back measurement, you'll subtract the back measurement from the front, to discover how much fabric you need to add.
To figure out how many extra rows of knitting you'll need to do in the bust dart zone, multiply the number of inches you need to add by the number of rows per inch in your gauge (if it's less than half an inch, it's not really enough to worry about, and you can quit while you're ahead). The trick is that you can't just add these rows any old where. And you can't actually let these rows go all the way to the seam line (much less all the way around).
Imagine for a moment, that you need to add two inches to the length of the front of your sweater. The back of your sweater is now 17 inches long. If you simply knit the front to be 19" long, you're going to have a bit of trouble sewing up the side seam. There will be this ugly pucker thing. And if you're doing it in the round... .. Nope, the trick is to add the length only in the middle (pretty much between your nipples), where you need it, while not adding at the side seam, where you don't.
That's where the next two measurements come in.
Okay, short rows. Right. But where do you start?
To figure that out, you'll need to know the distance from the top of your shoulder to the nipple (A-D) for top down garments (or the hem to the nipple (B-D) for bottom up garments). Part of the challenge is that it's not just a given number, since the total depth of the dart does impact the distance before the nipple line at which you'll start your dart. It's not supremely simple, but generally speaking, your figure that out by adding an inch to the depth of your dart, dividing that number in half, and starting that distance above (or below) your nipple line.
So, if you're adding 2" to the front of your sweater, you'd add an inch, getting 3". Divide that in half, and you got 1 1/2 inches. You'd start your dart 1 1/2" above (or below) your nipple line.
NOTE: This applies only when the additional length is all because of your bust! If you're also adding length to accommodate a belly (or if you're only adding for a belly because you're knitting this sweater for a beloved guy who likes beer too much), then these rules will change.
When you work the dart in our example, you'll work an inch worth of short rows, then and inch normally, and another inch of short rows.
Wait, short rows?
Yeah, short rows. And that's where that last measurement comes in. You'll need that extra length for the full width of your bust, but not the full width of our sweater. How do you do that?? By starting with your dart about an inch outside that nipple line.
To get there, you start with the distance between your nipples (D-D). To simplify where to start, divide that distance in half, and add and inch. So, if the distance between your nipples is 8", divide that in half to get 4", and add an inch, leaving you with 5". Then multiply that number by the number of stitches per inch your gauge gets for you. (So, if you're working with 5 st/in, you'll need 25 stitches.) What do you do with this number?
You make sure you know where the center line is for the front of your sweater (E), and start some short rows:
Once you've reached the row at which you'll start your short rows, work to the center marker (E), and then work the number of stitches you determined above (here it's 25 stitches) that will get you to one inch past D. Then, using your short row method of choice, wrap and turn (or turn and yo). Work back to the center line (E), and continue on for that same number of stitches past the center mark, wrap and turn (or turn and yo). For the next row, work to the turn, pick up the wrap (or yo) according to your preferred method, knit several* stitches more, and turn again. Repeat that last "row" until you've worked half the number of rows you need to add. You'll be happier with yourself in about an inch if you mark the place of the turning point on both sides. (*Note, that "several stitches" shoudl be the same number of stitches every time. Three will do just fine for aran weight, but you might want six or so for fingering).
Then work an inch even.
Next, reverse your short row shaping. So... work past the center until you reach the stitch at which you last turned, turn, and work past the center until you reach the stitch at which you last turned. Turn and work until whatever your "several stitches" were before the last turn. Repeat until you've worked the remaining number of rows you need to add. Voila, Pick up/conceal your wraps on the next round and you're done!
Suddenly, you've added two inches to the center of the front of your sweater. Voila, a 17" back and a 19" front... and the side seams MATCH.