Prior to the invention of the circular knitting needle, the only types of knitting needles available were rigid lengths of wood or metal in various diameters. The length of a needle was limited to about 13 inches because anything longer quickly became too heavy and unwieldy to be easily manipulated by the hands. These long needles typically had a point at one end and a “button” at the other to prevent stitches from sliding off, ranged in length from 8-13 inches and typically came in pairs.
For practical reasons, the width of any single piece of knitted fabric made on this type of needle was limited to the number of stitches you could fit onto the longest needle you could conveniently manipulate. Because of this, knitted garments had to be constructed “flat” — in the same way cloth garments were constructed — from pieces that were sewn together. For example, to make a cardigan sweater one knitted two sleeves, a left front, a right front, and a back, and sewed the pieces together. Consequently, historical knitting patterns, traditions, and techniques reflect this constraint.
The first circular revolution was the development of knitting in the round (ITR) by using short needles, typically from 6 to 8 inches long*, that have points on both ends ( double pointed needles or DPNs). The ITR technique made it possible to knit tubular garments like stockings and hose without seams. But again, for practical considerations, the circumference of the tube that can be knitted is limited by the number of double pointed needles you are willing to fiddle with — four being the practical limit. Even so, four DPNs are sufficient to hold enough stitches to accommodate the circumference of the largest foot (which is why DPNs come in sets of five). This technique also made it practical to knit gloves, mittens and hats without seams. (You could conceivably knit a seamless sweater on DPNs if (a) it was for a doll, or (b) you were willing to put up with the hassle of working with a garment on 6-15 DPNs. But, if you’ve ever used metal DPNs, you know how impractical it would be!)
With the advent of plastics came the second circular revolution, the invention of the circular knitting needle, first patented in 1918. This consists of two small single-pointed needles made of plastic, wood, or metal connected end-to-end by some flexible material such as nylon or plastic coated wire to make it, in effect, one long double pointed needle. Anyone who has used DPNs knows how frustratingly easy it is for stitches to slide off one end or the other of a DPN, or for a DPN to slide completely out and end up on the floor. The circular needle solves that problem. It also circumvents the width constraints of single-pointed needles as the length of the actual needle portion is only slightly wider than the palm of the hand, and the flexible cable allows the full weight of the knitted fabric to rest in the lap.
A circular needle can be anywhere from 9 inches (the shortest practical length) to 60 inches long. This wide variation in length makes it not only possible, but practical to knit tubular fabric with a circumference large enough to fit around chest or
hips and to knit very wide pieces of flat knitted fabric for making shawls, afghans and blankets without the need for piecing them together.
Necessity being the Mother of Invention**, knitters began employing the circular needle in new and inventive ways to do such things as knitting sleeves or socks two at a time using two fairly short circular needles, or using a single long circular needle in what is known as the Magic Loop technique.
For that matter, you can knit an entire sweater without seams on a single circular needle.
With the invention of the circular needle, knitting has become a three-dimensional craft, and patterns written after about 1960 begin to reflect this, moving toward more seamless construction techniques.
Now knitting has also entered the age of the internet: If there’s anything to do with knitting that you want to learn, somebody has posted a tutorial video about it on YouTube, and a glance through the patterns currently available on Ravelry makes it plain that if you can dream it, you can knit it.
*The makers of Shetland Lace use 13-15 inch long double pointed knitting needles in conjunction with a knitting stick, which is a neat trick if you can do it. **FYI, the Father of Invention is "There's got to be an easier way to do this!"