Picking up a knit selvedge

This is a step that I first tried two months ago. I was pleasantly surprised that it was far less difficult than I was anticipating.

Preparation

You'll need the following:
  • Needles smaller than the ones you intend to use, of the same type (straight, circular or double-pointed.)
  • Knowledge of your vertical gauge (rows/cm) in the material from which you're picking up and your horizontal gauge (stitches/cm) with the yarn and needles that you'll be using to knit new material.
    It is utterly vital that your gauge be over the same measurement. Further, this will be a lot easier if the number of stitches is a whole number. 3.5 rows / cm is much harder to work with than 7 rows / 2 cm.

Picking Up

For the sake of discussion, I'll presume that your stitches on the fabric from which you're picking up run left to right. That is to say that the material has been rotated ninety degrees clockwise. If it's turned the other way, reverse all lefts and right in the directions.

Looking at the fabric, the knit stitches should make "V"s, all pointing left. Directly above those should be your selvedge stitches. Between those, above the point of the "V", there is a hole in the fabric.

Insert one needle into this hole, then the other one beneath it. There should be two strands of yarn above the needles, as you're inserting the needles between two knit stitches. Knit one stitch.

Your new stitches should pull slightly to the right, making the hole through which you knit larger and more visible. Fortunately, as you pick up the other selvedge stitches, this becomes less visible.

Repeat this process with the next stitch, inserting the needle at the hole left of the knit "V" and knitting another stitch. This will give you one picked-up stitch per row.

Equalizing Sizes

There's a problem here, as you might guess from the gauge measurements. Namely, vertical gauge is different from horizontal gauge. There's two ways to solve that problem, one easy, the other giving more control over the fabric.

To cover the easy way first, simply knit the new material on needles where your horizontal gauge is the same as the vertical gauge on the fabric from which you're picking up. That is to say, if your vertical gauge is 5 rows / 2 cm, increase the needle size to ones where you have a horizontal gauge of 5 stitches / 2 cm.

Simple enough. The drawback is that, by necessity, the new fabric will be looser than the fabric from which you were picking up. If that doesn't bother you, carry on. If it does, read on.

To figure out how the fabric has to change requires a little math.

( stitches / cm ) / ( rows / cm ) == stitches / row

So, for example, if I have a horizontal gauge of 9 stitches / 2 cm and a vertical gauge of 7 rows / 2 cm, it works out as:

( 9 stitches / 2 cm ) / ( 7 rows / 2 cm ) == 9 / 7 stitches / row

The above is why I emphasized that the measurements need to be over the same distance. So long as your measurements are the same distance, the distances cancel out. It's also why the number of stitches need to be whole numbers: to avoid having fractions like 4.5 / 3.5. This is important because later on, these numbers are used for counting stitches, and 4.5 stitches is not a valid count.

If, on the other hand, I'm going from 24 rows / 5 cm to 8 stitches / 5 cm (small needles to larger ones), I get the calculation:

( 8 stitches / 5 cm ) / ( 24 rows / 5 cm ) == 8 / 24 stitches / row

Hopefully, it's evident that this part is simply creating a fraction with horizontal gauge over vertical gauge. There's nothing more to it if your measurement is the same.

If you want, reduce the fraction, so that both halves are as small as possible. This is only necessary for mathematical correctness (or ease of counting) though, so if you don't remember how or don't want to bother, leave it as it is.

Increasing

If the numerator (top part of the fraction) is greater than the denominator (bottom part of the fraction), that indicates that you need to increase the number of stitches to meet gauge.

Subtract the denominator from the numerator. The result is how many stitches you have to gain.

If I start with 9 / 7 stitches / row:

numerator - denominator == 9 - 7 == 2

So in that example, I need to gain 2 stitches over every 7. In other words, in every 7 stitches knit, I need two increases.

Decreasing

If the numerator (top part of the fraction) is less than the denominator (bottom part of the fraction), you'll need to decrease the number of stitches to meet gauge.

Subtract the numerator from the denominator. The result is how many stitches you need to lose. The result times 2 (because a decrease takes 2 stitches and makes them into 1 is how many stitches are required to make that many simple decreases.

If I start with 8 / 24 stitches / row:

denominator - numerator == 24 - 8 == 16

16 * 2 == 32

The result tells me that I need to lose 16 stitches, meaning that I need 32 stitches to decrease, or that I need to start rethinking needle sizes.

If I start with the more reasonable 7 / 9 stitches / row:

denominator - numerator == 9 - 7 == 2

2 * 2 == 4

This time, I need to lose two stitches every 9. That is to say, I need to decrease over 4 stitches, making them into 2. Because 4 is less than 9, this change in gauge is plausible.

Actually changing the number of stitches

The easiest way is to either knit or purl a row (depending on the rest of the pattern). Work all the increases or decreases on this row, which should sit close to the original fabric.

If you're working increases, do so with an eye to the bottom of the pattern, that way a stitch knit front and back can become two knit stitches on the next row.

In either case, it looks neater if the increases and decreases are spread out across the pattern, rather than being clustered together.

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