British needles are metrically sized for thickness - though you may, like me, have some ancient pairs which have old numbers on the knobs. If you buy from a UK charity shop you might get them with old numbers, too. Here is a conversion chart anyway - and I’ve given American sizes as well because they are different again!
Generally, for thinner yarn, use thinner needles. For double knitting yarn 3.25 and 4 will usually be specified in a pattern. For four ply, 2.75 and 3.25 are the norm. (In each case, the thinner pair of needles is used for ribbing.) If your tension is different from that given in your knitting pattern you can change needle size slightly to compensate. - not too much or you will change the texture.
You can get needles in different lengths. You may find longer ones easier for big, heavy pieces of work and shorter ones more comfortable for small items.
These are used for ‘knitting in the round’, which means fewer seams but is less common than using two straight needles. A circular needle is actually one long one, with two pointed rigid sections at either end and a flexible wire between. I have used them for baby shawls, as they can hold hundreds of stitches.
A set of four needles is often used for sock knitting - another way of knitting ‘in the round’ to avoid an uncomfortable seam. These are short needles with points at both ends. You divide your stitches among three needles and ‘knit’ with the fourth.
You really need one of these to knit cables or do any aran style knitting. They are short, double-pointed and sometimes have a bend in the middle. Pop a few stitches on temporarily at the front or back of the work, enabling you to ‘twist’ a few stitches into a diagonal or circle shape. It’s really easy but looks impressive!
Many knitters prefer the tactile feel of wood or bamboo but most needles are made of metal or plastic. I have found that bamboo does bend on heavy work.
Like a giant safety pin, it’s for holding some stitches while you work on the rest - for example, when you are knitting one shoulder piece of a jumper at a time. If you have only have a few stitches to ‘hold’ - like for a border - use an ordinary safety pin.
Row counter (tally)
A small plastic counter which slides on the end of a needle - push it right up near the ‘knob’. They come in a couple of sizes - to fit on thin and thick needles.