Updated: Jun 29, 2021
You finished your ribbing, you changed needles, and you're ready to tuck into the main fabric of your body. "Decreasing 1,567 stitches evenly across, work in st st." Cool.
Ideally, your pattern provides a rate of shaping
All exaggerations aside, why can't the pattern just tell you exactly what to do!? Sometimes, they do. My independently published patterns typically include specific counts for the decrease row. However, spelling it all out can be a huge use of space on the page and that doesn't work well for patterns going to print - and most patterns from design houses or publishing companies go to print. So you're likely to run across this instruction eventually!
Math for decreases
So, to decrease 15 stitches, we should divide our stitches by 16 (number of decreases plus one), so that the decreases are centered. This typically yields an answer with a remainder. To solve that you can play around with adding an extra few stitches at the beginning and/or in the middle of the piece.
Recently, I've been writing for a few publications that have style guide rules that require 'decrease evenly across.' As I knit my samples, I've been experimenting with doing this visually using removable stitch markers. Here's how:
Go ahead and divide your number of stitches by the number of decreases plus one, as above. Round the answer, this will be your stitches on either side of the first decreases.
At the beginning of the row, count in the number of stitches indicated and place a marker. At the end of the row, count in the number of stitches indicated plus two, and place a marker.
Visually begin to place markers, either dividing the fabric in half and then further halves, or roughly using the number you reached in step five. I like to place my markers in pairs, working from each side. It doesn't have to be perfect!
To work the decrease row, work to the first marker, remove the marker, dec. Repeat to the end.
Are you a visual learner? I've made a very amateur video for you on Jumprope, if you'd like step by step instructions!
General tips - transitioning from rib to main fabric
Technique tip - what kind of decrease should I make?
When you're transitioning from rib stitch to stockinette, your pattern might tell you how to decrease, or it might leave it up to you. In general, you'll do one of two things:
Decrease in stockinette - k2tog or p2tog, depending on which side of the work is facing, so that the decrease is a knit on the right side.
Decrease in pattern - k2tog or p2tog, depending on which stitch would be next in pattern.
Example: You're working in 1x1 rib and your last stitch was a purl. Decrease using a k2tog, the next stitch will also be a knit.
Example: You're working in 2x2 rib and your last two stitches were a knit then a purl. Your next stitch would normally be a purl, so p2tog; the next two stitches will be a knit followed by a purl.
Changing needle size
I've never seen a pattern list the gauge for ribbed trim. And yet, obviously, the designer has done the math to make sure your ribbing is the same width as, or smaller than, the main fabric of your sweater. There is an implicit gauge and it's a total mystery to you!
Often, that's fine. If a knitter has to go up or down to achieve gauge in the pattern for the main fabric, it's reasonable that they'll be successful if they go down the indicated number of needle sizes from the main fabric needle. But if you know that you knit stockinette on gauge but rib loosely, or knit stockinette loosely but rib with a death grip, consider adjusting your needle size for the rib.
To really truly nail your ribbing, just knit a swatch! This technique is also important if you don't have the budget to buy additional needles (ahem, me as a beginner knitter who assumed I could just 'knit tight' and wing it). Once you know your ribbing gauge, you can recalculate the number of stitches to work your ribbing and the revised number of decreases/increases needed to work the body.