Knitting is a method of creating a fabric by means of pulling rows of loops of yarn through other loops with straight needles. The fabric thus created is very elastic and well-suited to the creation of garments such as socks and hats which rely on elasticity to fit the wearer comfortably.
Knitting needles can be made of a variety of materials, including wood, metal, and plastic, and come in a variety of formats. Long needles with a point at one end and a stopper at the other are usually referred to as "straights", while those with a point at each end, used for knitting tubular fabrics, are called "double-pointeds" or "dpns". A special kind of needle consists of a flexible cable with a solid, pointed portion at each end; these are called "cable needles". Some types of knitting use needles with hooks at one end, much like crochet hooks.
Needles are sized by their diameter, which is important in determining the size of the stitches they will form. The size of the stitches, known as gauge or tension, is crucial to the size of the finished object, and needles are a vital part of determining gauge.
Aside from the needles, the only required tool is the yarn itself. While wool is probably the most popular fiber for knitting, many other materials are also used. Cotton, linen, silk, and many other synthetic and natural fibers are used for various purposes. Along with fiber content, yarn can also vary in weight, from bulky yarns that produce only a stitch or so to the inch to laceweight, which, on appropriate needles, produces 20 or more stitches per inch.
Most knitters also like to have a pair of scissors or some other method of cutting the yarn, a sewing needle of appropriate size for sewing sections of a garment together and hiding yarn ends, and a measuring tool to ensure accurate gauge. Some also use (often decorative) stitch and row markers, short cable needles for producing special effects, supplementary stitch holders to keep track of stitches not currently being worked, and other specialized tools. It's also usually handy to have some method of keeping track of one's place in a pattern, which can be as simple as a piece of paper with tick marks or as elaborate as a specialized electronic gadget programmed for the purpose.
To begin knitting, one must first cast on by putting the first row of loops on the needle. There are many methods of casting on, each with characteristic strengths and weaknesses. For example, a provisional cast on is useful if the knitter wishes to pick up stitches and work in the other direction at a later point, but cannot be left alone if she later changes her mind. Long-tail cast on, by contrast, is a permanent cast on and is flexible and neat, but requires careful judgement of the length of its eponymous tail lest one run out of yarn before the required number of stitches is reached.
The basic stitch in knitting is, unsurprisingly, called the knit stitch. To form it, the knitter inserts the tip of the right needle into the first loop on the left needle from front to back1, wraps the yarn around it, and pulls a new loop to the front. The old loop is dropped from the left needle and the process repeats until there are no loops remaining on the left needle. The purl stitch is very similar except that the right needle is inserted from back to front; a knit stitch on the front of the work is a purl stitch on the back, and vice versa2.
If she is working "flat", when the knitter reaches the end of a row of loops, the work is turned so that the needle with stitches on it is in the left hand again. If the knitter continues to knit every row, a fabric called garter stitch is produced. If she purls instead, and continues to alternate knitted and purled rows, the resultant fabric is called stockinette, and is what most people envision when they think of knitting: very smooth rows and columns of V-shaped stitches on one side, with the stitches of the other being quite bumpy and hard to distinguish.
Cable needles and double-pointed needles can be used to work "in the round" instead. In this case, the knitter sets up the initial cast on so that the last stitch is immediately followed by the first. This produces a tubular fabric. The beginning of the round must be marked in some way, commonly with a stitch marker hung from the needle or with a narrow, obvious stitch pattern. A knitter who wishes to produce stockinette in the round knits every stitch of every round; alternating knit and purl rounds makes garter stitch instead.
Knitters can produce additional stitches, and make the work wider, by increasing; stitches can be removed, thus narrowing the work, by decreasing. (In fact, in older patterns decreasing is often called "narrow" instead.) A special kind of increase, called a "yarn over", involves simply wrapping the yarn around the needle without inserting it into an already-formed stitch first. This leaves a deliberate hole in the fabric and is almost always used in lace patterns.
The well-known Aran sweater uses a technique called "cabling", in which several stitches (up to about six, depending on the weight of the yarn) are placed on a subsidiary, double-pointed needle and held to the front or back of the work. Some number of the stitches immediately following are then worked, and then the held stitches. This produces a small portion of the fabric in which stitches exchange places. Repetitions and elaborations of this method can create very intricate patterns indeed.
Knitting can be done with more than one strand of yarn; when the multiple strands are of different colors, the knitter can produce designs with them. In stranded colorwork, sometimes overarchingly (and erroneously) referred to as "Fair Isles", the strand not being used is simply carried along the back of the work. This makes for a thicker, and therefore warmer, fabric, which accounts for the technique's popularity in cold areas such as Scandinavia. Intarsia, on the other hand, requires the knitter to have a small bobbin of yarn for every block of color and to twist colors together where their sections meet. The classic argyle pattern is produced using an intarsia technique.
When finished, the work must be bound off so that the "live" stitches don't simply unravel. There are nearly as many ways to do this as there are to cast on; the simplest is to pass each stitch over the one following it until there is only one left, then pass the end of the yarn through the final loop. Care must be taken to not bind off too tightly.
There are naturally many elaborations of all of these basic techniques, which are beyond the scope of this article.
1: Some people teach left-handed knitters to knit from right to left instead, though this is generally considered inadvisable because knitting is essentially a two-handed activity and knitting right to left requires the knitter to reverse all the directions in standard patterns. However, in situations where rows are very short (such as knitting a narrow strip or when working entrelac), being able to knit "backwards" can save considerable time.
2: Because of the different orientation of the needle, some knitters' work is susceptible to a phenomenon known as "rowing out". This is caused by the tiny difference in the length of yarn necessary to circle the needle when purling as opposed to knitting, and can cause stitches on knit rows to be slightly shorter than those on purl rows. In stockinette stitch, rowing out is easily seen on both sides of the work and is considered a flaw. Knitters deal with the problem in different ways, including using a slightly smaller needle for purl rows, wrapping their yarns differently, pulling purl stitches a bit tighter, and working in the round so as to never have to purl at all.
The most pronounced difference of opinion in the knitting world is probably whether the yarn to be worked is held in the right hand or the left. These two methods are referred to respectively as "English" and "Continental", or as "throwing" and "picking" since the action of wrapping the yarn to form the stitch is rather more energetic in English knitting1. Knitting luminary Elizabeth Zimmerman favored the Continental style, and it is sometimes held to be more efficient and therefore faster, but opinions on the matter are extremely--and sometimes acrimoniously--divided. Those who do colorwork occasionally hold one color in each hand.
Less debated, but arguably more important from a structural point of view, is the division between "Eastern" and "Western". These two styles have to do with the direction in which the yarn is wrapped to form the stitch; a stitch formed using the Eastern method ends up with its "leading leg", the side closer to the working point, behind the needle, while Western stitches have their leading legs in front of the needle. Almost all patterns available in Europe and North America assume a Western style, which can cause problems for those who use the Eastern method; the mount of a stitch heavily influences which way a decrease will lean, which can be an important part of the look of many patterns. To add further confusion, some knitters use a "combined" method in which knits use the Eastern wrap while purls use the Western wrap2. Combining these methods incorrectly can lead to twisted stitches, which are not technically "bad", and may even lead to more elasticity than normal untwisted stitches, but have a slightly different look that may be unwanted.
Other regional variants include preferred methods of casting on and binding off, special techniques, fiber choices, and how the unworked yarn is held to properly tension it.
1: Though it is perfectly possible to throw with the yarn in the left hand, or to pick with the yarn in the right, the stereotypical methods are as above.
2: Apparently common in self-taught knitters, the combined method has the very useful characteristic of being far less susceptible to rowing out because the yarn is wrapped the same way to produce both knits and purls.