The welt is the lower edge of the gansey (or, to give it its technical name, “the bit at the bottom”). The most common welts were ribbed, with a simple knit 2/purl 2 rib, and this is the style I follow unless requested otherwise. (I like the look of them, and I like the fit.)
But another type of welt, the plain border, was common too, and some people prefer the look and feel of it. (You can see an example of a patterned plain welt here.) For this, you knit two sections (or flaps), each half the circumference of your gansey – one for the front and the other for the back. These are knit separately of each other, going back and forth, and with any pattern you like (traditionally they were usually just garter stitch, though some featured a ridged or other pattern) and as long as you like. When you’ve completed the two sections, you join them together when you knit the first round of the body, and carry on knitting in the round from there.
You can make a welt as long as you like – I’ve made them 4.5 inches long on occasion – but I find the length that works best is 3 or 3.5 inches. That’s deep enough to draw the ribbing in for a snug fit, and it looks well in proportion with the length of the body of the gansey, too. Less than 3 inches and you don’t get much “draw” and it hangs looser. But it’s all a matter of taste.
There’s only one other point I want to make about the welt. After the cast-on round, but before the ribbing, I like to knit a couple of rows of purl stitches. This isn’t necessary – in fact I can’t remember where I picked the habit up from, whether from a book or if I just imagined it – but I do find it useful. First of all, as I said above, when I cast on stitches they tend to come out a little uneven – some are big, others small – and I find a couple of rows of purl gives my knitting a chance to settle down before I begin what I think of as the gansey “proper”. Secondly, it makes it easier for me to see if there’s a hidden kink or twist in the stitches I’ve cast on, despite my best efforts to straighten them out. Thirdly, it’s a kind of relaxation after the strain of casting on (an activity which I don’t particularly enjoy!), just a couple of rows to unwind and let the tension out of my fingers. And finally, I quite like the effect it makes.
But, as I say, it’s not compulsory – and some people may not like the way it bulges out the ribbing at the bottom – so just go ahead with the ribbing right away if you prefer.